diff --git a/man7/system_data_types.7 b/man7/system_data_types.7
index b0e0d2b48..a4a2e0ab8 100644
--- a/man7/system_data_types.7
+++ b/man7/system_data_types.7
@@ -151,6 +151,28 @@ See also the
 .I float_t
 type in this page.
 .RE
+.\"------------------------------------- fd_set -----------------------/
+.TP
+.I fd_set
+.RS
+Include:
+.IR <sys/select.h> .
+Alternatively,
+.IR <sys/time.h> .
+.PP
+A structure type that can represent a set of file descriptors.
+According to POSIX,
+the maximum number of file descriptors in an
+.I fd_set
+structure is the value of the macro
+.BR FD_SETSIZE .
+.PP
+Conforming to:
+POSIX.1-2001 and later.
+.PP
+See also:
+.BR select (2)
+.RE
 .\"------------------------------------- fenv_t -----------------------/
 .TP
 .I fenv_t