I just cut Fido around his pee area , but i will propably cut up to his belly button when his coat grows longer.
Why do people cut under the armpits?

When he was a pup i cut him there, then he got so much matting there (when he was switching from puppy coat to normal), his coat is not so long now, when it gets longer and if it gets many matts then im gonna cut his hair under the armpits and longer up the stumach.