diff --git a/docs/pv.1 b/docs/pv.1
index b0acc1f..37d98c0 100644
--- a/docs/pv.1
+++ b/docs/pv.1
@@ -14,7 +14,7 @@ pv \- monitor and manage the progress of data through a pipe
 .PP
 \fBpv\fR \fB\-Q\fR|\fB\-\-query\fR \fIPID\fR [\fIOPTION\fR]...
 .PP
-\fBpv\fR [\fIOPTION\fR]... \fB\-M\fR|\fB\-\-monitor\fR \-\- \fICOMMAND\fR [\fIARGS\fR]...
+\fBpv\fR [\fIOPTION\fR]... \fB\-M\fR|\fB\-\-monitor\fR \fISIDE\fR \-\- \fICOMMAND\fR [\fIARGS\fR]...
 .\"
 .SH DESCRIPTION
 Show the progress of data through a pipeline by giving information such as
@@ -415,19 +415,31 @@ they match those of that process - such as
 \*(lq\fB\-\-average\-rate\-window\fR\*(rq.
 Data transfer modifiers will have no effect.
 .TP
-.B \-M, \-\-monitor
+.BR \-M " \fISIDE\fR, " \-\-monitor " \fISIDE\fR"
 Run the command specified by the remaining arguments, and monitor its
-standard input and standard output.
+standard input, its standard output, or both, depending on whether
+\fISIDE\fR is \*(lq\fBin\fR\*(rq, \*(lq\fBout\fR\*(rq, or
+\*(lq\fBboth\fR\*(rq.
 Use \*(lq\fB\-\-\fR\*(rq after all of the \fBpv\fR options to keep the
 monitored command's options separate.
 .IP
-For example, \*(lq\fBpv\~\-\-cursor\~\-\-monitor\~\-\-\~gzip\~\-9\fR\*(rq is
+With a \fISIDE\fR of \*(lq\fBboth\fR\*(rq, two progress bars will be shown.
+All of \fBpv\fR's display switches, output modifiers, and data transfer
+modifiers will apply to both sides.
+The \*(lq\fB\-\-format\fR\*(rq and \*(lq\fB\-\-name\fR\*(rq options may be
+specified twice, in which case the first one applies to the input side and
+the second applies to the output side.
+.IP
+For example,
+\*(lq\fBpv\~\-\-cursor\~\-\-monitor\~both\~\-\-\~gzip\~\-9\fR\*(rq is
 equivalent to
 \*(lq\fBpv\~\-\-cursor\~|\~gzip\~\-9\~|\~pv\~\-\-cursor\fR\*(rq, except that
 the latter can't show the input:output ratio.
-.\"TODO: choice of displaying two progress bars (in + out) or only one
+.IP
+The values \*(lq\fB0\fR\*(rq, \*(lq\fB1\fR\*(rq, and \*(lq\fB2\fR\*(rq may
+be used as synonyms for \*(lq\fBin\fR\*(rq, \*(lq\fBout\fR\*(rq, and
+\*(lq\fBboth\fR\*(rq.
 .\"TODO: formatting option for input:output ratio
-.\"TODO: a way to specify the name of each bar, if there are two
 .\"
 .SS "Other options"
 .TP
diff --git a/src/main/help.c b/src/main/help.c
index 07837e9..5664bee 100644
--- a/src/main/help.c
+++ b/src/main/help.c
@@ -372,8 +372,8 @@ void display_help(void)
 		 N_("show progress of process PID"),
 		 { 0, 0, 0, 0} },
 #endif				/* PV_REMOTE_CONTROL */
-		{ "-M", "--monitor", NULL,
-		 N_("monitor the input and output of a command"),
+		{ "-M", "--monitor", N_("SIDE"),
+		 N_("run a command and monitor its standard input, output, or both"),
 		 { 0, 0, 0, 0} },
 		{ "", NULL, NULL, NULL, { 0, 0, 0, 0} },
 		{ "-P", "--pidfile", N_("FILE"),
diff --git a/src/main/options.c b/src/main/options.c
index ce3b192..dc77b4f 100644
--- a/src/main/options.c
+++ b/src/main/options.c
@@ -680,7 +680,7 @@ opts_t opts_parse(unsigned int argc, char **argv)
 		{ "watchfd", 1, NULL, (int) 'd' },
 		{ "output", 1, NULL, (int) 'o' },
 		{ "average-rate-window", 1, NULL, (int) 'm' },
-		{ "monitor", 0, NULL, (int) 'M' },
+		{ "monitor", 1, NULL, (int) 'M' },
 #ifdef ENABLE_DEBUGGING
 		{ "debug", 1, NULL, (int) '!' },
 #endif				/* ENABLE_DEBUGGING */
@@ -689,7 +689,7 @@ opts_t opts_parse(unsigned int argc, char **argv)
 	/*@+nullassign@ */
 	int option_index = 0;
 #endif				/* HAVE_GETOPT_LONG */
-	char *short_options = "hVpteIrab8kTA:fvnqcWD:s:gl0i:w:H:N:u:F:x:L:B:CEZ:SYKOXU:R:Q:P:d:m:o:M"
+	char *short_options = "hVpteIrab8kTA:fvnqcWD:s:gl0i:w:H:N:u:F:x:L:B:CEZ:SYKOXU:R:Q:P:d:m:o:M:"
 #ifdef ENABLE_DEBUGGING
 	    "!:"
 #endif
@@ -996,6 +996,7 @@ opts_t opts_parse(unsigned int argc, char **argv)
 			opts->height_set_manually = opts->height == 0 ? false : true;
 			break;
 		case 'N':
+			/* TODO: preserve previous name for use with "-M both" */
 			opts->name = pv_strdup(optarg);
 			if (NULL == opts->name) {
 				fprintf(stderr, "%s: -N: %s\n", opts->program_name, strerror(errno));
@@ -1070,6 +1071,7 @@ opts_t opts_parse(unsigned int argc, char **argv)
 			}
 			break;
 		case 'F':
+			/* TODO: preserve previous format for use with "-M both" */
 			opts->format = pv_strdup(optarg);
 			if (NULL == opts->format) {
 				fprintf(stderr, "%s: -F: %s\n", opts->program_name, strerror(errno));
@@ -1105,6 +1107,7 @@ opts_t opts_parse(unsigned int argc, char **argv)
 			break;
 		case 'M':
 			opts->action = PV_ACTION_MONITOR;
+			/* TODO: parse optarg to set the side */
 			break;
 #ifdef ENABLE_DEBUGGING
 		case '!':
@@ -1259,8 +1262,7 @@ opts_t opts_parse(unsigned int argc, char **argv)
 	 */
 	if ((PV_ACTION_MONITOR == opts->action) && (optind >= (int) argc)) {
 		/*@-mustfreefresh@ *//* see above */
-		fprintf(stderr, "%s: -M: %s\n", opts->program_name,
-			_("a command to run must be specified"));
+		fprintf(stderr, "%s: -M: %s\n", opts->program_name, _("a command to run must be specified"));
 		opts_free(opts);
 		return NULL;
 		/*@+mustfreefresh@ */