diff --git a/src/imu/calibrate.rs b/src/imu/calibrate.rs
index 9966728..167b33e 100644
--- a/src/imu/calibrate.rs
+++ b/src/imu/calibrate.rs
@@ -7,7 +7,8 @@
 //! - **Down** from the two level stretches, parked facing one way ("imu level A") and
 //!   turned round ("imu level B"). Averaged, a slope's tilt cancels, since turning
 //!   round reverses it; the difference between them is reported as the slope.
-//!   One stretch alone still works, slope included.
+//!   One stretch alone still works, slope included. A session with neither keeps the
+//!   down of the current mount file, so level and forward can be captured on different days.
 //! - **Forward** from straight-line pull-aways ("imu pull away", forward) and stops
 //!   ("imu brake", backward): the force's horizontal part, signed by which stretch it
 //!   is in, summed. Without them, forward is the `--forward` axis laid flat, as before.
@@ -101,7 +102,12 @@ pub struct Calibration {
 }
 
 /// `fallback_forward`: the configured forward axis, used when no driving was captured.
-pub fn calibrate(samples: &[Sample], stretches: &[Stretch], fallback_forward: [f64; 3]) -> Result<Calibration, String> {
+pub fn calibrate(
+    samples: &[Sample],
+    stretches: &[Stretch],
+    fallback_forward: [f64; 3],
+    fallback_down: Option<[f64; 3]>,
+) -> Result<Calibration, String> {
     let has = |s: &Stretch, k: &str| s.label.contains(k);
 
     // Down: the mean force while still is "up"; down is its opposite.
@@ -120,8 +126,15 @@ pub fn calibrate(samples: &[Sample], stretches: &[Stretch], fallback_forward: [f
         }
         ups.push(unit(m));
     }
+    let mut down_from = "the level stretches".to_string();
     let up = match ups.as_slice() {
-        [] => return Err("no 'imu level A' or 'imu level B' stretch with samples".into()),
+        [] => match fallback_down {
+            Some(d) => {
+                down_from = "the current mount file (no level stretch in this session)".into();
+                unit(d.map(|x| -x))
+            }
+            None => return Err("no 'imu level A' or 'imu level B' stretch with samples, and no mount file to take down from".into()),
+        },
         [a] => *a,
         [a, b, ..] => unit([a[0] + b[0], a[1] + b[1], a[2] + b[2]]),
     };
@@ -170,6 +183,7 @@ pub fn calibrate(samples: &[Sample], stretches: &[Stretch], fallback_forward: [f
         report: json!({
             "forward": r3(rows[0]), "right": r3(rows[1]), "down": r3(rows[2]),
             "level": level,
+            "down_from": down_from,
             "slope_deg": slope.map(|x| (x * 10.0).round() / 10.0),
             "forward_from": forward_from,
             "forward_spread_deg": spread.map(|x| (x * 10.0).round() / 10.0),
@@ -206,7 +220,7 @@ mod tests {
             st("29 imu pull away #1", 40.0, 43.0),
             st("30 imu brake #1", 50.0, 53.0),
         ];
-        let r = calibrate(&samples, &stretches, [0.0, 1.0, 0.0]).unwrap();
+        let r = calibrate(&samples, &stretches, [0.0, 1.0, 0.0], None).unwrap();
         let rows = r.mount.rows();
         let close = |a: [f64; 3], b: [f64; 3]| (0..3).all(|i| (a[i] - b[i]).abs() < 1e-6);
         assert!(close(rows[2], [-1.0, 0.0, 0.0]), "down {:?}", rows[2]);
@@ -220,11 +234,15 @@ mod tests {
     #[test]
     fn without_driving_forward_falls_back_and_motion_is_refused() {
         let samples = steady(0.0, 10.0, [0.0, 0.0, 1.0]);
-        let r = calibrate(&samples, &[st("27 imu level A", 0.0, 10.0)], [0.0, 1.0, 0.0]).unwrap();
+        let r = calibrate(&samples, &[st("27 imu level A", 0.0, 10.0)], [0.0, 1.0, 0.0], None).unwrap();
         assert!(r.report["forward_from"].as_str().unwrap().starts_with("the --forward axis"));
         let mut moving = steady(0.0, 10.0, [0.0, 0.0, 1.0]);
         moving.iter_mut().for_each(|s| s.gyro = [0.0, 0.0, 20.0]);
-        assert!(calibrate(&moving, &[st("27 imu level A", 0.0, 10.0)], [0.0, 1.0, 0.0]).is_err());
-        assert!(calibrate(&samples, &[], [0.0, 1.0, 0.0]).is_err());
+        assert!(calibrate(&moving, &[st("27 imu level A", 0.0, 10.0)], [0.0, 1.0, 0.0], None).is_err());
+        assert!(calibrate(&samples, &[], [0.0, 1.0, 0.0], None).is_err());
+        // Forward only, down from an earlier calibration.
+        let drive = steady(0.0, 3.0, [0.0, 0.3, 1.0]);
+        let r = calibrate(&drive, &[st("30 imu brake #1", 0.0, 3.0)], [0.0, 1.0, 0.0], Some([0.0, 0.0, -1.0])).unwrap();
+        assert!((r.mount.rows()[0][1] + 1.0).abs() < 1e-6, "{}", r.report); // braking force +Y: forward is -Y
     }
 }
diff --git a/src/imu/main.rs b/src/imu/main.rs
index 8e5495a..76eeb41 100644
--- a/src/imu/main.rs
+++ b/src/imu/main.rs
@@ -223,7 +223,7 @@ fn calibrate_session(o: &Opts, dir: &str) -> ExitCode {
     let dir = std::path::Path::new(dir);
     let result = calibrate::load_samples(dir)
         .and_then(|s| Ok((s, calibrate::load_stretches(dir)?)))
-        .and_then(|(s, st)| calibrate::calibrate(&s, &st, o.forward.unit()));
+        .and_then(|(s, st)| calibrate::calibrate(&s, &st, o.forward.unit(), mount_file(&o.mount_file).map(|(m, _)| m.rows()[2])));
     let c = match result {
         Ok(c) => c,
         Err(e) => {