diff --git a/libvips/iofuncs/operation.c b/libvips/iofuncs/operation.c
index 23a330da..2e4c01ca 100644
--- a/libvips/iofuncs/operation.c
+++ b/libvips/iofuncs/operation.c
@@ -176,13 +176,23 @@
  * Flags we associate with an operation.
  *
  * @VIPS_OPERATION_SEQUENTIAL means that the operation works like vips_conv():
- * it can happily process images top-to-bottom with only small non-local
- * references.
+ * it can process images top-to-bottom with only small non-local
+ * references. 
+ *
+ * Every scan-line must be requested, you are not allowed to skip
+ * ahead, but as a special case, the very first request can be for a region
+ * not at the top of the image. In this case, the first part of the image will
+ * be read and discarded
  *
  * @VIPS_OPERATION_SEQUENTIAL_UNBUFFERED means that the operation works like 
- * vips_copy(): it can happily process images top-to-bottom and makes no 
+ * vips_copy(): it can process images top-to-bottom and makes no 
  * non-local references. 
  *
+ * Every scan-line must be requested, you are not allowed to skip
+ * ahead, but as a special case, the very first request can be for a region
+ * not at the top of the image. In this case, the first part of the image will
+ * be read and discarded
+ *
  * @VIPS_OPERATION_NOCACHE means that the operation must not be cached by
  * vips. 
  *