[Freesurfer] glm question

Dear FreeSurfer experts,

I'm doing some group analyses and I'm a little stocked with this:

I have Six Groups (2 Factors, one with three levels and the other one with two levels), and one covariate. I don't know the rule for setting the contrasts for an ANOVA and for the interaction between the two factors regressing out the covariate. I've checked at the FS wiki and found the following contrast for six groups (two factors) and zero covariates.

Contrast 6 gender-x-handedness.mtx (interaction between gender and handedness)

[0.333 -0.333 -0.333 0.333 0.000 0.000 0.000 0.000 0.000]

[0.000 0.000 0.000 0.000 0.000 0.333 -0.333 -0.333 0.333]

I don’t understand the rule that has been followed.

Can any one help me with this?

Bests,

Gabriel