diff --git a/common/rfb/Decoder.h b/common/rfb/Decoder.h
index 4195a80..7286b41 100644
--- a/common/rfb/Decoder.h
+++ b/common/rfb/Decoder.h
@@ -38,9 +38,15 @@
 
     // readRect() transfers data for the given rectangle from the
     // InStream to the OutStream, possibly changing it along the way to
-    // make it easier to decode.
+    // make it easier to decode. This function will always be called in
+    // a serial manner on the main thread.
     virtual void readRect(const Rect& r, rdr::InStream* is,
                           const ConnParams& cp, rdr::OutStream* os)=0;
+
+    // These functions will be called from any of the worker threads.
+    // A lock will be held whilst these are called so it is safe to
+    // read and update internal state as necessary.
+
     // decodeRect() decodes the given rectangle with data from the
     // given buffer, onto the ModifiablePixelBuffer. The PixelFormat of
     // the PixelBuffer might not match the ConnParams and it is up to
@@ -49,6 +55,7 @@
                             size_t buflen, const ConnParams& cp,
                             ModifiablePixelBuffer* pb)=0;
 
+  public:
     static bool supported(int encoding);
     static Decoder* createDecoder(int encoding);
   };