The twentieth letter of the modern English alphabet and the nineteenth of the ancient Roman one, corresponding to Greek tau, Hebrew taw.
To a T means exactly, to perfection. Recorded from the late 17th century, this idiom may reflect the idea of completing the letter T by putting in the cross-stroke. However, an earlier expression in the same sense was to a title, so T may be an abbreviation. Attempts to link it with a golfer's tee or a builder's T-square are unconvincing.