What letter does Mrs. Joe write to Biddy after being struck in the back of the head in Great Expectations?
Maybe I have always thought about this too figuratively, but I thought the oddly shaped T or J to be a hammer or a mallet to divulge a little information about what had happened to her.
Biddy remembers when talking to Pip one day,
'Why of course!' cried Biddy, with an exultant face. 'Don't you see? It's him.
Orlick, without a doubt. She had lost his name, and could only signify him by his hammer.
Mrs. Joe was trying to tell Biddy how this happened to her. This occured in the very end of chapter 16. A few paragraphs from the bottom are worth reading again to know for sure what happened.
I think what you are referring to is in Chapter 16. You may be having problems because you might think she has written a letter in the sense of something that starts "Dear Biddy..."
But she hasn't written that kind of a letter. She has written a single letter (like A, B, C) on a slate. Before Biddy came to stay with them, Mrs. Joe had been writing something that looked like "a curious T." Pip can't figure out what it means.
But when Biddy comes, she understands it pretty quickly. It is meant as a symbol for Joe and so it is probably supposed to be a J.