diff --git a/docs/G-Codes.md b/docs/G-Codes.md
index 026b93ec..37e363f4 100644
--- a/docs/G-Codes.md
+++ b/docs/G-Codes.md
@@ -755,9 +755,13 @@ is enabled (also see the
 - `TEST_RESONANCES AXIS=<axis> OUTPUT=<resonances,raw_data>
   [NAME=<name>] [FREQ_START=<min_freq>] [FREQ_END=<max_freq>]
   [HZ_PER_SEC=<hz_per_sec>] [INPUT_SHAPING=[<0:1>]]`: Runs the resonance
-  test in all configured probe points for the requested axis (X or Y)
+  test in all configured probe points for the requested <axis>
   and measures the acceleration using the accelerometer chips configured
-  for the respective axis. If `INPUT_SHAPING=0` or not set (default),
+  for the respective axis. <axis> can either be X or Y, or specify an
+  arbitrary direction as `AXIS=dx,dy`, where dx and dy are floating point
+  numbers defining a direction vector (e.g. `AXIS=X`, `AXIS=Y`, or
+  `AXIS=1,-1` to define a diagonal direction). Note that `AXIS=dx,dy` and
+  `AXIS=-dx,-dy` is equivalent. If `INPUT_SHAPING=0` or not set (default),
   disables input shaping for the resonance testing, because it is not valid
   to run the resonance testing with the input shaper enabled.
   `OUTPUT` parameter is a comma-separated list of which outputs will be
diff --git a/docs/Measuring_Resonances.md b/docs/Measuring_Resonances.md
index 557857c4..8b7fed0f 100644
--- a/docs/Measuring_Resonances.md
+++ b/docs/Measuring_Resonances.md
@@ -357,6 +357,35 @@ If you are doing a shaper re-calibration and the reported smoothing for the
 suggested shaper configuration is almost the same as what you got during the
 previous calibration, this step can be skipped.
 
+## Testing custom axes
+
+`TEST_RESONANCES` command supports custom axes. While this is not really
+useful for input shaper calibration, it can be used to study printer
+resonances in-depth and to check, for example, belt tension.
+
+To check the belt tension on CoreXY printers, execute
+```
+TEST_RESONANCES AXIS=1,1 OUTPUT=raw_data
+TEST_RESONANCES AXIS=1,-1 OUTPUT=raw_data
+```
+and use `graph_accelerometer.py` to process the generated files, e.g.
+```
+~/klipper/scripts/graph_accelerometer.py -c /tmp/raw_data_axis*.csv -o /tmp/resonances.png
+```
+which will generate `/tmp/resonances.png` comparing the resonances.
+
+For Delta printers with the default tower placement
+(tower A ~= 210 degrees, B ~= 330 degrees, and C ~= 90 degrees), execute
+```
+TEST_RESONANCES AXIS=0,1 OUTPUT=raw_data
+TEST_RESONANCES AXIS=-0.866025404,-0.5 OUTPUT=raw_data
+TEST_RESONANCES AXIS=0.866025404,-0.5 OUTPUT=raw_data
+```
+and then use the same command
+```
+~/klipper/scripts/graph_accelerometer.py -c /tmp/raw_data_axis*.csv -o /tmp/resonances.png
+```
+to generate `/tmp/resonances.png` comparing the resonances.
 
 # Input Shaper auto-calibration