How to measure an armhole?

I’m knitting a cardigan for my mother-in-law to be and I just realized that I haven’t a clue on how to measure an armhole properly. Can anyone help me??

Don’t measure around the curve. Imagine two parallel lines…shoulder and underarm. The distance between those two.

cam

Or in other words, measure straight up from the row you bound off the sts on.

That is so helpful! I have only been knitting for a few months and I have been mostly doing very simple projects with minimal shaping. Thank you for your help.