USMLE Gout/Pseudogout crystals and birefringence

[IK3]

Wherever I look about birefringence of gout or pseudogout all I find is negative or positive birefrigence, respectively. If you want to get into more details on how to identify it on the step it says parallel is yellow and perpendicular is blue when it comes to MSU in gout and vice versa in CaPP. The part that frustrates me is that I can't find is how to identify the axis of the compensator? I mean if someone is referring an object being parallel or perpendicular it has to be in reference to an axis... not sure if it is such a basic concept that no bothers to mention and I just zoned out in class when it was covered . Anyway hopefully someone can shed some light on this.

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[mybluedog]

This was some time ago. But the axis of the compensator is the red line

gout=- bi= yellow crystals are parallel/blue is perpendicular to the red compensators

pseudogout=+bi=rhomboid shape (this really should be the big diff you see) the blue becomes parallel instead of perpendicular