diff --git a/src/io/io.go b/src/io/io.go
index db88125f50..9d4c0d2506 100644
--- a/src/io/io.go
+++ b/src/io/io.go
@@ -111,7 +111,8 @@ type Closer interface {
 // interpreted according to whence:
 // SeekStart means relative to the start of the file,
 // SeekCurrent means relative to the current offset, and
-// SeekEnd means relative to the end.
+// SeekEnd means relative to the end
+// (for example, offset = -2 specifies the penultimate byte of the file).
 // Seek returns the new offset relative to the start of the
 // file or an error, if any.
 //