diff --git a/libs/ardour/ardour/segment_descriptor.h b/libs/ardour/ardour/segment_descriptor.h
new file mode 100644
index 0000000000..08fdd3ec57
--- /dev/null
+++ b/libs/ardour/ardour/segment_descriptor.h
@@ -0,0 +1,79 @@
+/*
+ * Copyright (C) 2021 Paul Davis <paul@linuxaudiosystems.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef __libardour_segment_descriptor_h__
+#define __libardour_segment_descriptor_h__
+
+#include "temporal/timeline.h"
+#include "temporal/tempo.h"
+
+namespace ARDOUR {
+
+/* An object that describes an extent (duration & position), along with a
+ * potentially expanding set of metadata about that extent (e.g. bpm, meter
+ * etc.)
+ */
+
+class SegmentDescriptor {
+public:
+	SegmentDescriptor ();
+
+	/* This object does not use the tempo map to convert between time
+	 * domains, since it describes things that are not (always) on the
+	 * timeline.
+	 */
+
+	Temporal::TimeDomain time_domain() const { return _time_domain; }
+
+	void set_position (Temporal::samplepos_t);
+	void set_position (Temporal::Beats const &);
+
+	void set_duration (Temporal::samplecnt_t);
+	void set_duration (Temporal::Beats const &);
+
+	void set_extent (Temporal::samplepos_t pos, Temporal::samplecnt_t dur);
+	void set_extent (Temporal::Beats const & pos, Temporal::Beats const & dur);
+
+	Temporal::timecnt_t extent() const;
+	Temporal::timepos_t position() const { return extent().position(); }
+
+	Temporal::Tempo tempo() const { return _tempo; }
+	void set_tempo (Temporal::Tempo const&);
+
+	Temporal::Meter meter() const { return _meter; }
+	void set_meter (Temporal::Meter const&);
+
+private:
+	Temporal::TimeDomain _time_domain;
+
+	/* cannot use a union for these because Temporal::Beats has a
+	   "non-trivial" constructor.
+	*/
+
+	Temporal::samplepos_t _position_samples;
+	Temporal::Beats       _position_beats;
+	Temporal::samplepos_t _duration_samples;
+	Temporal::Beats       _duration_beats;
+
+	Temporal::Tempo _tempo;
+	Temporal::Meter _meter;
+};
+
+}
+
+#endif /* __libardour_segment_descriptor_h__ */
diff --git a/libs/ardour/segment_descriptor.cc b/libs/ardour/segment_descriptor.cc
new file mode 100644
index 0000000000..c2ba20ba27
--- /dev/null
+++ b/libs/ardour/segment_descriptor.cc
@@ -0,0 +1,109 @@
+/*
+ * Copyright (C) 2021 Paul Davis <paul@linuxaudiosystems.com>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "ardour/segment_descriptor.h"
+
+using namespace ARDOUR;
+using namespace Temporal;
+
+SegmentDescriptor::SegmentDescriptor ()
+	: _time_domain (AudioTime)
+	, _position_samples (0)
+	, _duration_samples (0)
+	, _tempo (120, 4)
+	, _meter (4, 4)
+{
+}
+
+void
+SegmentDescriptor::set_position (samplepos_t s)
+{
+	if (_time_domain != AudioTime) {
+		/* XXX error */
+		return;
+	}
+	_position_samples = s;
+}
+
+void
+SegmentDescriptor::set_position (Temporal::Beats const & b)
+{
+	if (_time_domain != BeatTime) {
+		/* XXX error */
+		return;
+	}
+	_position_beats = b;
+}
+
+void
+SegmentDescriptor::set_duration (samplepos_t s)
+{
+	if (_time_domain != AudioTime) {
+		/* XXX error */
+		return;
+	}
+	_position_samples = s;
+}
+
+void
+SegmentDescriptor::set_duration (Temporal::Beats const & b)
+{
+	if (_time_domain != BeatTime) {
+		/* XXX error */
+		return;
+	}
+	_duration_beats = b;
+}
+
+void
+SegmentDescriptor::set_extent (Temporal::Beats const & p, Temporal::Beats const & d)
+{
+	_time_domain = BeatTime;
+	_position_beats = p;
+	_duration_beats = d;
+}
+
+void
+SegmentDescriptor::set_extent (samplepos_t p, samplecnt_t d)
+{
+	_time_domain = AudioTime;
+	_position_samples = p;
+	_duration_samples = d;
+}
+
+timecnt_t
+SegmentDescriptor::extent() const
+{
+	if (_time_domain == BeatTime) {
+		return timecnt_t (_duration_beats, timepos_t (_position_beats));
+	}
+
+	return timecnt_t (_duration_samples, timepos_t (_position_samples));
+}
+
+void
+SegmentDescriptor::set_tempo (Tempo const & t)
+{
+	_tempo = t;
+}
+
+void
+SegmentDescriptor::set_meter (Meter const & m)
+{
+	_meter = m;
+}
diff --git a/libs/ardour/wscript b/libs/ardour/wscript
index 4be9caae82..9e721aee2c 100644
--- a/libs/ardour/wscript
+++ b/libs/ardour/wscript
@@ -208,6 +208,7 @@ libardour_sources = [
         'rt_tasklist.cc',
         'scene_change.cc',
         'search_paths.cc',
+        'segment_descriptor.cc',
         'selection.cc',
         'send.cc',
         'session.cc',