diff --git a/glymur/jp2box.py b/glymur/jp2box.py
index 462fbe8..ee3e973 100644
--- a/glymur/jp2box.py
+++ b/glymur/jp2box.py
@@ -160,7 +160,7 @@ class Jp2kBox(object):
         fptr.write(struct.pack('>I', end_pos - orig_pos))
         fptr.seek(end_pos)
 
-    def _parse_this_box(self, fptr, box_id, start, num_bytes, box_is_XL):
+    def _parse_this_box(self, fptr, box_id, start, num_bytes):
         """Parse the current box.
 
         Parameters
@@ -172,10 +172,7 @@ class Jp2kBox(object):
             4-letter identifier for the current box.
         start, num_bytes : int
             Byte offset and length of the current box.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
+
         Returns
         -------
         box : Jp2kBox
@@ -195,7 +192,7 @@ class Jp2kBox(object):
             return box
 
         try:
-            box = parser(fptr, start, num_bytes, box_is_XL=box_is_XL)
+            box = parser(fptr, start, num_bytes)
         except ValueError as err:
             msg = "Encountered an unrecoverable ValueError while parsing a {0} "
             msg += "box at byte offset {1}.  The original error message was "
@@ -236,7 +233,6 @@ class Jp2kBox(object):
                 warnings.warn(msg)
                 return superbox
 
-            box_is_XL = False
             (box_length, box_id) = struct.unpack('>I4s', read_buffer)
             if box_length == 0:
                 # The length of the box is presumed to last until the end of
@@ -247,14 +243,12 @@ class Jp2kBox(object):
                 # The length of the box is in the XL field, a 64-bit value.
                 read_buffer = fptr.read(8)
                 num_bytes, = struct.unpack('>Q', read_buffer)
-                box_is_XL = True
 
             else:
                 # The box_length value really is the length of the box!
                 num_bytes = box_length
 
-            box = self._parse_this_box(fptr, box_id, start, num_bytes,
-                                       box_is_XL)
+            box = self._parse_this_box(fptr, box_id, start, num_bytes)
 
             superbox.append(box)
 
@@ -410,7 +404,7 @@ class ColourSpecificationBox(Jp2kBox):
         fptr.write(read_buffer)
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse JPEG 2000 color specification box.
 
         Parameters
@@ -421,16 +415,12 @@ class ColourSpecificationBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         ColourSpecificationBox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         read_buffer = fptr.read(num_bytes)
         # Read the brand, minor version.
         (method, precedence, approximation) = struct.unpack_from('>BBB',
@@ -666,7 +656,7 @@ class ChannelDefinitionBox(Jp2kBox):
                                    self.association[j]))
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse component definition box.
 
         Parameters
@@ -677,16 +667,12 @@ class ChannelDefinitionBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         ComponentDefinitionBox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         read_buffer = fptr.read(num_bytes)
 
         # Read the number of components.
@@ -740,7 +726,7 @@ class CodestreamHeaderBox(Jp2kBox):
         self._write_superbox(fptr, b'jpch')
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse codestream header box.
 
         Parameters
@@ -751,10 +737,6 @@ class CodestreamHeaderBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -813,7 +795,7 @@ class ColourGroupBox(Jp2kBox):
         self._write_superbox(fptr, b'cgrp')
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse colour group box.
 
         Parameters
@@ -824,10 +806,6 @@ class ColourGroupBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -880,7 +858,7 @@ class CompositingLayerHeaderBox(Jp2kBox):
         self._write_superbox(fptr, b'jplh')
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse compositing layer header box.
 
         Parameters
@@ -891,10 +869,6 @@ class CompositingLayerHeaderBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -975,7 +949,7 @@ class ComponentMappingBox(Jp2kBox):
             fptr.write(write_buffer)
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse component mapping box.
 
         Parameters
@@ -986,16 +960,12 @@ class ComponentMappingBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         ComponentMappingBox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         num_components = int(num_bytes/4)
 
         read_buffer = fptr.read(num_bytes)
@@ -1049,7 +1019,7 @@ class ContiguousCodestreamBox(Jp2kBox):
         return msg
 
     @classmethod
-    def parse(cls, fptr, offset=0, length=0, box_is_XL=False):
+    def parse(cls, fptr, offset=0, length=0):
         """Parse a codestream box.
 
         Parameters
@@ -1060,10 +1030,6 @@ class ContiguousCodestreamBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -1151,7 +1117,7 @@ class DataReferenceBox(Jp2kBox):
         return msg
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse data reference box.
 
         Parameters
@@ -1162,16 +1128,12 @@ class DataReferenceBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         DataReferenceBox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         read_buffer = fptr.read(num_bytes)
 
         # Read the number of data references
@@ -1281,7 +1243,7 @@ class FileTypeBox(Jp2kBox):
             fptr.write(item.encode())
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse JPEG 2000 file type box.
 
         Parameters
@@ -1292,16 +1254,12 @@ class FileTypeBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         FileTypeBox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         read_buffer = fptr.read(num_bytes)
         # Extract the brand, minor version.
         (brand, minor_version) = struct.unpack_from('>4sI', read_buffer, 0)
@@ -1398,7 +1356,7 @@ class FragmentListBox(Jp2kBox):
             fptr.write(write_buffer)
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse JPX free box.
 
         Parameters
@@ -1409,16 +1367,12 @@ class FragmentListBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         FragmentListBox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         read_buffer = fptr.read(num_bytes)
         num_fragments, = struct.unpack_from('>H', read_buffer, offset=0)
 
@@ -1461,7 +1415,7 @@ class FragmentTableBox(Jp2kBox):
         return msg
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse JPX fragment table superbox box.
 
         Parameters
@@ -1472,10 +1426,6 @@ class FragmentTableBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -1536,7 +1486,7 @@ class FreeBox(Jp2kBox):
         return msg
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse JPX free box.
 
         Parameters
@@ -1547,10 +1497,6 @@ class FreeBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -1664,7 +1610,7 @@ class ImageHeaderBox(Jp2kBox):
         fptr.write(read_buffer)
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse JPEG 2000 image header box.
 
         Parameters
@@ -1675,10 +1621,6 @@ class ImageHeaderBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -1736,7 +1678,7 @@ class AssociationBox(Jp2kBox):
         return msg
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse association box.
 
         Parameters
@@ -1747,10 +1689,6 @@ class AssociationBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -1806,7 +1744,7 @@ class JP2HeaderBox(Jp2kBox):
         self._write_superbox(fptr, b'jp2h')
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse JPEG 2000 header box.
 
         Parameters
@@ -1817,10 +1755,6 @@ class JP2HeaderBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -1878,7 +1812,7 @@ class JPEG2000SignatureBox(Jp2kBox):
         fptr.write(struct.pack('>BBBB', *self.signature))
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse JPEG 2000 signature box.
 
         Parameters
@@ -1889,10 +1823,6 @@ class JPEG2000SignatureBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -2000,7 +1930,7 @@ class PaletteBox(Jp2kBox):
                 fptr.write(write_buffer)
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse palette box.
 
         Parameters
@@ -2011,16 +1941,12 @@ class PaletteBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         PaletteBox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         read_buffer = fptr.read(num_bytes)
         nrows, ncols = struct.unpack_from('>HB', read_buffer, offset=0)
 
@@ -2232,7 +2158,7 @@ class ReaderRequirementsBox(Jp2kBox):
         return msg
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse reader requirements box.
 
         Parameters
@@ -2243,16 +2169,12 @@ class ReaderRequirementsBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         ReaderRequirementsBox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         read_buffer = fptr.read(num_bytes)
         mask_length, = struct.unpack_from('>B', read_buffer, offset=0)
 
@@ -2446,7 +2368,7 @@ class ResolutionBox(Jp2kBox):
         return msg
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse Resolution box.
 
         Parameters
@@ -2457,10 +2379,6 @@ class ResolutionBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -2514,7 +2432,7 @@ class CaptureResolutionBox(Jp2kBox):
         return msg
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse CaptureResolutionBox.
 
         Parameters
@@ -2525,10 +2443,6 @@ class CaptureResolutionBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -2581,7 +2495,7 @@ class DisplayResolutionBox(Jp2kBox):
         return msg
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse display resolution box.
 
         Parameters
@@ -2592,10 +2506,6 @@ class DisplayResolutionBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -2653,7 +2563,7 @@ class LabelBox(Jp2kBox):
         fptr.write(self.label.encode())
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse Label box.
 
         Parameters
@@ -2664,16 +2574,12 @@ class LabelBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         LabelBox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         read_buffer = fptr.read(num_bytes)
         label = read_buffer.decode('utf-8')
         return cls(label, length=length, offset=offset)
@@ -2728,7 +2634,7 @@ class NumberListBox(Jp2kBox):
         return msg
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse number list box.
 
         Parameters
@@ -2739,16 +2645,12 @@ class NumberListBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         LabelBox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         raw_data = fptr.read(num_bytes)
         num_associations = int(len(raw_data) / 4)
         lst = struct.unpack('>' + 'I' * num_associations, raw_data)
@@ -2836,7 +2738,7 @@ class XMLBox(Jp2kBox):
         fptr.write(read_buffer)
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse XML box.
 
         Parameters
@@ -2847,16 +2749,12 @@ class XMLBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         XMLBox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         read_buffer = fptr.read(num_bytes)
         try:
             text = read_buffer.decode('utf-8')
@@ -2940,7 +2838,7 @@ class UUIDListBox(Jp2kBox):
         return msg
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse UUIDList box.
 
         Parameters
@@ -2951,16 +2849,12 @@ class UUIDListBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         UUIDListBox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         read_buffer = fptr.read(num_bytes)
 
         num_uuids, = struct.unpack_from('>H', read_buffer)
@@ -3004,7 +2898,7 @@ class UUIDInfoBox(Jp2kBox):
         return msg
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse UUIDInfo super box.
 
         Parameters
@@ -3015,10 +2909,6 @@ class UUIDInfoBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
@@ -3101,7 +2991,7 @@ class DataEntryURLBox(Jp2kBox):
         return msg
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse data entry URL box.
 
         Parameters
@@ -3112,16 +3002,12 @@ class DataEntryURLBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         DataEntryURLbox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         read_buffer = fptr.read(num_bytes)
         data = struct.unpack_from('>BBBB', read_buffer)
         version = data[0]
@@ -3278,7 +3164,7 @@ class UUIDBox(Jp2kBox):
         fptr.write(self.raw_data)
 
     @classmethod
-    def parse(cls, fptr, offset, length, box_is_XL=False):
+    def parse(cls, fptr, offset, length):
         """Parse UUID box.
 
         Parameters
@@ -3289,16 +3175,12 @@ class UUIDBox(Jp2kBox):
             Start position of box in bytes.
         length : int
             Length of the box in bytes.
-        box_is_XL : bool
-            True if the box has an XL field.  In this case, the start of the
-            box is 16 bytes behind the start of the box payload instead of being
-            8 bytes behind.
 
         Returns
         -------
         UUIDBox instance
         """
-        num_bytes = length - 16 if box_is_XL else length - 8
+        num_bytes = offset + length - fptr.tell()
         read_buffer = fptr.read(num_bytes)
         the_uuid = uuid.UUID(bytes=read_buffer[0:16])
         return cls(the_uuid, read_buffer[16:], length=length, offset=offset)