Why do we say “grammatically” instead of “grammarically”?

Question

My son said the word “grammarically”, and I corrected him that the word is actually “grammatically”. He asked why that’s the case since it’s referencing grammar and not grammat. I could not find the answer via google. Please help.

In: Other

Answer 1

basically when the adjective ends in “ic” it gets the -ally ending:

Basic – basically

Gramatic – grammatically