diff --git a/cmd-new-session.c b/cmd-new-session.c
index deda88f2..08fc6065 100644
--- a/cmd-new-session.c
+++ b/cmd-new-session.c
@@ -68,12 +68,12 @@ cmd_new_session_exec(struct cmd *self, struct cmdq_item *item)
 {
 	struct args		*args = self->args;
 	struct client		*c = item->client;
-	struct session		*s, *as;
-	struct session		*groupwith = item->state.tflag.s;
+	struct session		*s, *as, *groupwith;
 	struct window		*w;
 	struct environ		*env;
 	struct termios		 tio, *tiop;
-	const char		*newname, *target, *errstr, *template;
+	struct session_group	*sg;
+	const char		*newname, *errstr, *template, *group, *prefix;
 	const char		*path, *cmd, *cwd, *to_free = NULL;
 	char		       **argv, *cause, *cp;
 	int			 detached, already_attached, idx, argc;
@@ -119,13 +119,29 @@ cmd_new_session_exec(struct cmd *self, struct cmdq_item *item)
 		}
 	}
 
-	if ((target = args_get(args, 't')) != NULL) {
+	/* Is this going to be part of a session group? */
+	group = args_get(args, 't');
+	if (group != NULL) {
+		groupwith = item->state.tflag.s;
 		if (groupwith == NULL) {
-			cmdq_error(item, "no such session: %s", target);
-			goto error;
-		}
-	} else
+			if (!session_check_name(group)) {
+				cmdq_error(item, "bad group name: %s", group);
+				goto error;
+			}
+			sg = session_group_find(group);
+		} else
+			sg = session_group_contains(groupwith);
+		if (sg != NULL)
+			prefix = sg->name;
+		else if (groupwith != NULL)
+			prefix = groupwith->name;
+		else
+			prefix = group;
+	} else {
 		groupwith = NULL;
+		sg = NULL;
+		prefix = NULL;
+	}
 
 	/* Set -d if no client. */
 	detached = args_has(args, 'd');
@@ -213,7 +229,7 @@ cmd_new_session_exec(struct cmd *self, struct cmdq_item *item)
 	if (!args_has(args, 't') && args->argc != 0) {
 		argc = args->argc;
 		argv = args->argv;
-	} else if (groupwith == NULL) {
+	} else if (sg == NULL && groupwith == NULL) {
 		cmd = options_get_string(global_s_options, "default-command");
 		if (cmd != NULL && *cmd != '\0') {
 			argc = 1;
@@ -239,8 +255,8 @@ cmd_new_session_exec(struct cmd *self, struct cmdq_item *item)
 
 	/* Create the new session. */
 	idx = -1 - options_get_number(global_s_options, "base-index");
-	s = session_create(newname, argc, argv, path, cwd, env, tiop, idx, sx,
-	    sy, &cause);
+	s = session_create(prefix, newname, argc, argv, path, cwd, env, tiop,
+	    idx, sx, sy, &cause);
 	environ_free(env);
 	if (s == NULL) {
 		cmdq_error(item, "create session failed: %s", cause);
@@ -259,8 +275,15 @@ cmd_new_session_exec(struct cmd *self, struct cmdq_item *item)
 	 * If a target session is given, this is to be part of a session group,
 	 * so add it to the group and synchronize.
 	 */
-	if (groupwith != NULL) {
-		session_group_add(groupwith, s);
+	if (group != NULL) {
+		if (sg == NULL) {
+			if (groupwith != NULL) {
+				sg = session_group_new(groupwith->name);
+				session_group_add(sg, groupwith);
+			} else
+				sg = session_group_new(group);
+		}
+		session_group_add(sg, s);
 		session_group_synchronize_to(s);
 		session_select(s, RB_MIN(winlinks, &s->windows)->idx);
 	}
diff --git a/cmd-swap-window.c b/cmd-swap-window.c
index 0a93fa6e..38252745 100644
--- a/cmd-swap-window.c
+++ b/cmd-swap-window.c
@@ -52,11 +52,11 @@ cmd_swap_window_exec(struct cmd *self, struct cmdq_item *item)
 
 	wl_src = item->state.sflag.wl;
 	src = item->state.sflag.s;
-	sg_src = session_group_find(src);
+	sg_src = session_group_contains(src);
 
 	wl_dst = item->state.tflag.wl;
 	dst = item->state.tflag.s;
-	sg_dst = session_group_find(dst);
+	sg_dst = session_group_contains(dst);
 
 	if (src != dst && sg_src != NULL && sg_dst != NULL &&
 	    sg_src == sg_dst) {
diff --git a/format.c b/format.c
index 145c31f5..4df17025 100644
--- a/format.c
+++ b/format.c
@@ -1097,10 +1097,10 @@ format_defaults_session(struct format_tree *ft, struct session *s)
 	format_add(ft, "session_height", "%u", s->sy);
 	format_add(ft, "session_id", "$%u", s->id);
 
-	sg = session_group_find(s);
+	sg = session_group_contains(s);
 	format_add(ft, "session_grouped", "%d", sg != NULL);
 	if (sg != NULL)
-		format_add(ft, "session_group", "%u", session_group_index(sg));
+		format_add(ft, "session_group", "%s", sg->name);
 
 	format_add_tv(ft, "session_created", &s->creation_time);
 	format_add_tv(ft, "session_last_attached", &s->last_attached_time);
diff --git a/server-fn.c b/server-fn.c
index d43461f3..c14b4cab 100644
--- a/server-fn.c
+++ b/server-fn.c
@@ -78,7 +78,7 @@ server_redraw_session_group(struct session *s)
 {
 	struct session_group	*sg;
 
-	if ((sg = session_group_find(s)) == NULL)
+	if ((sg = session_group_contains(s)) == NULL)
 		server_redraw_session(s);
 	else {
 		TAILQ_FOREACH(s, &sg->sessions, gentry)
@@ -102,7 +102,7 @@ server_status_session_group(struct session *s)
 {
 	struct session_group	*sg;
 
-	if ((sg = session_group_find(s)) == NULL)
+	if ((sg = session_group_contains(s)) == NULL)
 		server_status_session(s);
 	else {
 		TAILQ_FOREACH(s, &sg->sessions, gentry)
@@ -220,7 +220,7 @@ server_kill_window(struct window *w)
 		}
 
 		if (options_get_number(s->options, "renumber-windows")) {
-			if ((sg = session_group_find(s)) != NULL) {
+			if ((sg = session_group_contains(s)) != NULL) {
 				TAILQ_FOREACH(target_s, &sg->sessions, gentry)
 					session_renumber_windows(target_s);
 			} else
@@ -238,8 +238,8 @@ server_link_window(struct session *src, struct winlink *srcwl,
 	struct winlink		*dstwl;
 	struct session_group	*srcsg, *dstsg;
 
-	srcsg = session_group_find(src);
-	dstsg = session_group_find(dst);
+	srcsg = session_group_contains(src);
+	dstsg = session_group_contains(dst);
 	if (src != dst && srcsg != NULL && dstsg != NULL && srcsg == dstsg) {
 		xasprintf(cause, "sessions are grouped");
 		return (-1);
@@ -348,7 +348,7 @@ server_destroy_session_group(struct session *s)
 	struct session_group	*sg;
 	struct session		*s1;
 
-	if ((sg = session_group_find(s)) == NULL)
+	if ((sg = session_group_contains(s)) == NULL)
 		server_destroy_session(s);
 	else {
 		TAILQ_FOREACH_SAFE(s, &sg->sessions, gentry, s1) {
diff --git a/server.c b/server.c
index 176cafcb..0ce90db6 100644
--- a/server.c
+++ b/server.c
@@ -156,8 +156,7 @@ server_start(struct event_base *base, int lockfd, char *lockfile)
 	RB_INIT(&all_window_panes);
 	TAILQ_INIT(&clients);
 	RB_INIT(&sessions);
-	TAILQ_INIT(&session_groups);
-	mode_key_init_trees();
+	RB_INIT(&session_groups);
 	key_bindings_init();
 
 	gettimeofday(&start_time, NULL);
diff --git a/session.c b/session.c
index 09c2dc26..5ffa2b87 100644
--- a/session.c
+++ b/session.c
@@ -42,6 +42,9 @@ static void	session_group_remove(struct session *);
 static u_int	session_group_count(struct session_group *);
 static void	session_group_synchronize1(struct session *, struct session *);
 
+static u_int	session_group_count(struct session_group *);
+static void	session_group_synchronize1(struct session *, struct session *);
+
 RB_GENERATE(sessions, session, entry, session_cmp);
 
 int
@@ -50,6 +53,14 @@ session_cmp(struct session *s1, struct session *s2)
 	return (strcmp(s1->name, s2->name));
 }
 
+RB_GENERATE(session_groups, session_group, entry, session_group_cmp);
+
+int
+session_group_cmp(struct session_group *s1, struct session_group *s2)
+{
+	return (strcmp(s1->name, s2->name));
+}
+
 /*
  * Find if session is still alive. This is true if it is still on the global
  * sessions list.
@@ -107,9 +118,9 @@ session_find_by_id(u_int id)
 
 /* Create a new session. */
 struct session *
-session_create(const char *name, int argc, char **argv, const char *path,
-    const char *cwd, struct environ *env, struct termios *tio, int idx,
-    u_int sx, u_int sy, char **cause)
+session_create(const char *prefix, const char *name, int argc, char **argv,
+    const char *path, const char *cwd, struct environ *env, struct termios *tio,
+    int idx, u_int sx, u_int sy, char **cause)
 {
 	struct session	*s;
 	struct winlink	*wl;
@@ -150,7 +161,10 @@ session_create(const char *name, int argc, char **argv, const char *path,
 		do {
 			s->id = next_session_id++;
 			free(s->name);
-			xasprintf(&s->name, "%u", s->id);
+			if (prefix != NULL)
+				xasprintf(&s->name, "%s-%u", prefix, s->id);
+			else
+				xasprintf(&s->name, "%u", s->id);
 		} while (RB_FIND(sessions, &sessions, s) != NULL);
 	}
 	RB_INSERT(sessions, &sessions, s);
@@ -429,7 +443,7 @@ session_is_linked(struct session *s, struct window *w)
 {
 	struct session_group	*sg;
 
-	if ((sg = session_group_find(s)) != NULL)
+	if ((sg = session_group_contains(s)) != NULL)
 		return (w->references != session_group_count(sg));
 	return (w->references != 1);
 }
@@ -540,12 +554,12 @@ session_set_current(struct session *s, struct winlink *wl)
 
 /* Find the session group containing a session. */
 struct session_group *
-session_group_find(struct session *target)
+session_group_contains(struct session *target)
 {
 	struct session_group	*sg;
 	struct session		*s;
 
-	TAILQ_FOREACH(sg, &session_groups, entry) {
+	RB_FOREACH(sg, session_groups, &session_groups) {
 		TAILQ_FOREACH(s, &sg->sessions, gentry) {
 			if (s == target)
 				return (sg);
@@ -554,39 +568,39 @@ session_group_find(struct session *target)
 	return (NULL);
 }
 
-/* Find session group index. */
-u_int
-session_group_index(struct session_group *sg)
+/* Find session group by name. */
+struct session_group *
+session_group_find(const char *name)
 {
-	struct session_group   *sg2;
-	u_int			i;
+	struct session_group	sg;
 
-	i = 0;
-	TAILQ_FOREACH(sg2, &session_groups, entry) {
-		if (sg == sg2)
-			return (i);
-		i++;
-	}
-
-	fatalx("session group not found");
+	sg.name = name;
+	return (RB_FIND(session_groups, &session_groups, &sg));
 }
 
-/*
- * Add a session to the session group containing target, creating it if
- * necessary.
- */
-void
-session_group_add(struct session *target, struct session *s)
+/* Create a new session group. */
+struct session_group *
+session_group_new(const char *name)
 {
 	struct session_group	*sg;
 
-	if ((sg = session_group_find(target)) == NULL) {
-		sg = xmalloc(sizeof *sg);
-		TAILQ_INSERT_TAIL(&session_groups, sg, entry);
-		TAILQ_INIT(&sg->sessions);
-		TAILQ_INSERT_TAIL(&sg->sessions, target, gentry);
-	}
-	TAILQ_INSERT_TAIL(&sg->sessions, s, gentry);
+	if ((sg = session_group_find(name)) != NULL)
+		return (sg);
+
+	sg = xcalloc(1, sizeof *sg);
+	sg->name = xstrdup(name);
+	TAILQ_INIT(&sg->sessions);
+
+	RB_INSERT(session_groups, &session_groups, sg);
+	return (sg);
+}
+
+/* Add a session to a session group. */
+void
+session_group_add(struct session_group *sg, struct session *s)
+{
+	if (session_group_contains(s) == NULL)
+		TAILQ_INSERT_TAIL(&sg->sessions, s, gentry);
 }
 
 /* Remove a session from its group and destroy the group if empty. */
@@ -595,13 +609,11 @@ session_group_remove(struct session *s)
 {
 	struct session_group	*sg;
 
-	if ((sg = session_group_find(s)) == NULL)
+	if ((sg = session_group_contains(s)) == NULL)
 		return;
 	TAILQ_REMOVE(&sg->sessions, s, gentry);
-	if (TAILQ_NEXT(TAILQ_FIRST(&sg->sessions), gentry) == NULL)
-		TAILQ_REMOVE(&sg->sessions, TAILQ_FIRST(&sg->sessions), gentry);
 	if (TAILQ_EMPTY(&sg->sessions)) {
-		TAILQ_REMOVE(&session_groups, sg, entry);
+		RB_REMOVE(session_groups, &session_groups, sg);
 		free(sg);
 	}
 }
@@ -626,7 +638,7 @@ session_group_synchronize_to(struct session *s)
 	struct session_group	*sg;
 	struct session		*target;
 
-	if ((sg = session_group_find(s)) == NULL)
+	if ((sg = session_group_contains(s)) == NULL)
 		return;
 
 	target = NULL;
@@ -634,7 +646,8 @@ session_group_synchronize_to(struct session *s)
 		if (target != s)
 			break;
 	}
-	session_group_synchronize1(target, s);
+	if (target != NULL)
+		session_group_synchronize1(target, s);
 }
 
 /* Synchronize a session group to a session. */
@@ -644,7 +657,7 @@ session_group_synchronize_from(struct session *target)
 	struct session_group	*sg;
 	struct session		*s;
 
-	if ((sg = session_group_find(target)) == NULL)
+	if ((sg = session_group_contains(target)) == NULL)
 		return;
 
 	TAILQ_FOREACH(s, &sg->sessions, gentry) {
diff --git a/tmux.1 b/tmux.1
index a712519d..d4f5a891 100644
--- a/tmux.1
+++ b/tmux.1
@@ -812,7 +812,7 @@ Lock all clients attached to
 .Op Fl F Ar format
 .Op Fl n Ar window-name
 .Op Fl s Ar session-name
-.Op Fl t Ar target-session
+.Op Fl t Ar group-name
 .Op Fl x Ar width
 .Op Fl y Ar height
 .Op Ar shell-command
@@ -857,16 +857,27 @@ to
 .Pp
 If
 .Fl t
-is given, the new session is
-.Em grouped
-with
-.Ar target-session .
-This means they share the same set of windows - all windows from
-.Ar target-session
-are linked to the new session, any new windows are linked to both sessions and
-any windows closed removed from both sessions.
+is given, it specifies a
+.Ic session group .
+Sessions in the same group share the same set of windows - new windows are
+linked to all sessions in the grouo and any windows closed removed from all
+sessions.
 The current and previous window and any session options remain independent and
-either session may be killed without affecting the other.
+any session in a group may be killed without affecting the others.
+The
+.Ar group-name
+argument may be:
+.Bl -enum -width Ds
+.It
+the name of an existing group, in which case the new session is added to that
+group;
+.It
+the name of an existing session - the new session is added to the same group
+as that session, creating a new group if necessary;
+.It
+the name for a new group containing only the new session.
+.El
+.Pp
 .Fl n
 and
 .Ar shell-command
@@ -3562,7 +3573,7 @@ The following variables are available, where appropriate:
 .It Li "session_activity" Ta "" Ta "Integer time of session last activity"
 .It Li "session_created" Ta "" Ta "Integer time session created"
 .It Li "session_last_attached" Ta "" Ta "Integer time session last attached"
-.It Li "session_group" Ta "" Ta "Number of session group"
+.It Li "session_group" Ta "" Ta "Name of session group"
 .It Li "session_grouped" Ta "" Ta "1 if session in a group"
 .It Li "session_height" Ta "" Ta "Height of session"
 .It Li "session_id" Ta "" Ta "Unique session ID"
diff --git a/tmux.h b/tmux.h
index 09c980df..2a5dbe65 100644
--- a/tmux.h
+++ b/tmux.h
@@ -901,11 +901,12 @@ struct environ_entry {
 
 /* Client session. */
 struct session_group {
-	TAILQ_HEAD(, session) sessions;
+	const char		*name;
+	TAILQ_HEAD(, session)	 sessions;
 
-	TAILQ_ENTRY(session_group) entry;
+	RB_ENTRY(session_group)	 entry;
 };
-TAILQ_HEAD(session_groups, session_group);
+RB_HEAD(session_groups, session_group);
 
 struct session {
 	u_int		 id;
@@ -2210,13 +2211,15 @@ extern struct sessions sessions;
 extern struct session_groups session_groups;
 int	session_cmp(struct session *, struct session *);
 RB_PROTOTYPE(sessions, session, entry, session_cmp);
+int	session_group_cmp(struct session_group *, struct session_group *);
+RB_PROTOTYPE(session_groups, session_group, entry, session_group_cmp);
 int		 session_alive(struct session *);
 struct session	*session_find(const char *);
 struct session	*session_find_by_id_str(const char *);
 struct session	*session_find_by_id(u_int);
-struct session	*session_create(const char *, int, char **, const char *,
-		     const char *, struct environ *, struct termios *, int,
-		     u_int, u_int, char **);
+struct session	*session_create(const char *, const char *, int, char **,
+		     const char *, const char *, struct environ *,
+		     struct termios *, int, u_int, u_int, char **);
 void		 session_destroy(struct session *);
 void		 session_unref(struct session *);
 int		 session_check_name(const char *);
@@ -2235,9 +2238,10 @@ int		 session_previous(struct session *, int);
 int		 session_select(struct session *, int);
 int		 session_last(struct session *);
 int		 session_set_current(struct session *, struct winlink *);
-struct session_group *session_group_find(struct session *);
-u_int		 session_group_index(struct session_group *);
-void		 session_group_add(struct session *, struct session *);
+struct session_group *session_group_contains(struct session *);
+struct session_group *session_group_find(const char *);
+struct session_group *session_group_new(const char *);
+void		 session_group_add(struct session_group *, struct session *);
 void		 session_group_synchronize_to(struct session *);
 void		 session_group_synchronize_from(struct session *);
 void		 session_renumber_windows(struct session *);