Index: usr.bin/ar/ar.1
===================================================================
--- usr.bin/ar/ar.1
+++ usr.bin/ar/ar.1
@@ -222,8 +222,8 @@
 .Fl U
 options are specified on the command line, the final one takes precedence.
 .It Fl f
-Synonymous with option
-.Fl T .
+Use only the first fifteen characters of the archive member name or
+command line file name argument when naming archive members.
 .It Fl i Ar member-before
 Synonymous with option
 .Fl b .
@@ -314,8 +314,14 @@
 in the order in which they appear in the archive, one per line.
 If no files are specified, all files in the archive are listed.
 .It Fl T
-Use only the first fifteen characters of the archive member name or
-command line file name argument when naming archive members.
+Not supported.
+In earlier versions of
+.Nm .
+it was an alias for
+.Fl f .
+In other implementations of
+.Nm
+it creates a "thin" archive.
 .It Fl u
 Conditionally update the archive or extract members.
 When used with the
Index: usr.bin/ar/ar.c
===================================================================
--- usr.bin/ar/ar.c
+++ usr.bin/ar/ar.c
@@ -66,6 +66,7 @@
 #include <sys/queue.h>
 #include <sys/types.h>
 #include <archive.h>
+#include <err.h>
 #include <errno.h>
 #include <getopt.h>
 #include <libgen.h>
@@ -193,7 +194,6 @@
 			Uflag = 0;
 			break;
 		case 'f':
-		case 'T':
 			bsdar->options |= AR_TR;
 			break;
 		case 'j':
@@ -226,6 +226,9 @@
 		case 's':
 			bsdar->options |= AR_S;
 			break;
+		case 'T':
+			errx(1, "-T option is not supported");
+			break;
 		case 't':
 			set_mode(bsdar, opt);
 			break;