diff --git a/src/client/sync/io_sync.mli b/src/client/sync/io_sync.mli
index 303a099e..883efaa0 100644
--- a/src/client/sync/io_sync.mli
+++ b/src/client/sync/io_sync.mli
@@ -1,2 +1,4 @@
+(** Synchronous IOs, with threads for concurrency *)
+
 include Generic_io.S_WITH_CONCURRENCY with type 'a t = 'a
 (** Generic IO with [spawn] starting a background thread *)