Confusion between pitch/roll and huge drift

Hi, there are a couple problems I'm wondering about:
1. The tutorial doesn't seem to mention which axis is pitch and which is roll. From the values, rotation along the long axis seem to be pitch, the short axis seem to be roll. Is that the case(I would imagine the other way around)? Would be nice to make a illustration to clarify.
2. The pitch value is getting a very huge drift after I rotates the board around for a short while. If I rotates it for a round, it sits at +-1.0

Start by reading the tutorial section that defines pitch and roll, then reproduce the reported drift by rotating the board. Confirm the axis mapping, clarify it with an illustration, and establish whether the pitch value still reaches approximately ±1.0 after one rotation.

Written by the indexing model from the issue text.