diff --git a/panda/src/express/subStream.I b/panda/src/express/subStream.I
index 835cdffe4c..b801fb6339 100644
--- a/panda/src/express/subStream.I
+++ b/panda/src/express/subStream.I
@@ -35,6 +35,9 @@ ISubStream() : istream(&_buf) {
 //               start total characters.  The character at "end"
 //               within the source will never be read; this will
 //               appear to be EOF.
+//
+//               If end is zero, it indicates that the ISubStream will
+//               continue until the end of the source stream.
 ////////////////////////////////////////////////////////////////////
 INLINE void ISubStream::
 open(istream *source, streampos start, streampos end) {
@@ -46,7 +49,7 @@ open(istream *source, streampos start, streampos end) {
 //     Function: ISubStream::close
 //       Access: Public
 //  Description: Resets the SubStream to empty, but does not actually
-//               close its source istream.
+//               close the source istream.
 ////////////////////////////////////////////////////////////////////
 INLINE void ISubStream::
 close() {
diff --git a/panda/src/express/subStream.h b/panda/src/express/subStream.h
index 59d5fa5501..05fba9b088 100644
--- a/panda/src/express/subStream.h
+++ b/panda/src/express/subStream.h
@@ -29,6 +29,10 @@
 //               stream will be the "start" character from the source
 //               istream; just before the file pointer reaches the
 //               "end" character, eof is returned.
+//
+//               The source stream must be one that we can randomly
+//               seek within.  The resulting ISubStream will also
+//               support arbitrary seeks.
 ////////////////////////////////////////////////////////////////////
 class EXPCL_PANDAEXPRESS ISubStream : public istream {
 public: