Has a dog ever been taught to recognize words?

I’ve got my girl, Frolich, sitting on my lap and resting her head on the keyboard as I try to work : )

But that got me thinking and wondering. Dogs have been able to learn mathematic problems, and are well known for being able to take word-based commands.

So I began to wonder if it was possible to, theoretically or in actuality, to teach a dog some written words?
Like, teaching them SIT means “sit” or something?
And how far do you think it could go, if you think it’s possible?