Fits To a T

According the the website Phrases.org.uk

Here's what I have found out about 'Fit to a T'

If something "fits to a T" then it's perfect for its purpose. The allusion here is said to be with a T square. This piece of apparatus is so accurate that a precise right angle fits it perfectly.
However neat this suggestion is, there is another possible origin, based on the fact that the saying was in use in the 17th century, before the T square was invented. This one suggests that the T stands for "Title", a minute and precisely positioned pen stroke or printer's mark. A tiny brushstroke was all that distinguished the Hebrew letter "dalet" from "resh". "Title" was the word chosen by Wycliffe to translate references to this tiny difference in his version of the New Testament. Thus the mark was perfectly suited to its task.

We've had this discussion before, and some have expressed scepticism about the history of the T sqaure, as not being that old.