diff --git a/docs/CPU_DATA_ANALYSIS.md b/docs/CPU_DATA_ANALYSIS.md
index ff749ff6d..077bdefe8 100644
--- a/docs/CPU_DATA_ANALYSIS.md
+++ b/docs/CPU_DATA_ANALYSIS.md
@@ -629,7 +629,7 @@ For oscillation/rotation data, each image corresponds to a rotation angle $\phi$
 $
 \mathbf{s}_\mathrm{obs,ref} = R(\phi)\,\mathbf{s}_\mathrm{obs},
 $
-with $R(\phi)$ constructed from the axis-angle representation of the goniometer model. The angle $\phi$ is taken at the centre of each frame's oscillation (the frame angle plus half the oscillation width).
+where $R(\phi)$ is the rotation by $+\phi$ about the goniometer axis **as stored in the file**. The sign is a convention and it is load-bearing: rotating the observations forward by $+\phi$ means the crystal itself turns by $-\phi$ about that stored axis, i.e. $R(\phi)$ is the *inverse* of the crystal's own rotation from the reference orientation to frame $\phi$. The same convention is why the unmerged-MTZ batch headers and the XDS geometry echo carry the axis **negated** relative to the input file ([rugnux ▸ the unmerged export](RUGNUX.md#the-unmerged-export)) — a reimplementation that takes $R(\phi)$ as the crystal rotation must use $R(-\phi)$ here instead. The angle $\phi$ is taken at the centre of each frame's oscillation (the frame angle plus half the oscillation width).
 
 ### 7.4 Multi-stage tightening of inlier tolerance