In Chapter 16, why does Mrs. Joe keep writing T's?

Biddy quickly assesses that Mrs. Jow writes the letter T to ask for Orlick. Her injury had left her at a loss for his name, thus, she wrote the letter T in its place. The T represented a hammer.

"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.