Whats the difference in usage between cait and chat?
The basic word for cat is cat. So far so good.
When it is lenited it becomes chat. So far so French.
"Cait" is the unlenited plural. "Cats." So far, so Lloyd-Weber.
'S e do bheatha.
You had to apply some logic here. Both cat 'cat' and cait 'cats' would fit as there was nothing to tell you if the word was singular or plural. But there was no reason for the c to lenite to ch so you had to choose the cait.