We use the expression “to a T” very commonly in the sense of minute exactness, perfection; as, the coat fits to a T; the meat was done to a T.
It is easy to dismiss the origin of the expression, as, we am sorry to say, some of our leading dictionaries do, by attributing it to the draftsman’s T-square, which is supposed to be an exact instrument, but the evidence indicates that the expression was in common English use before the T-square got its name.
“To a T” dates back to the seventeenth century in literary use and was undoubtedly common in everyday speech long before any writer dared to or thought to use it in print. But it is likely that the name of the instrument, “T-square,” would have been in print shortly after its invention, yet the first mention is in the eighteenth century.
The sense of the expression corresponds, however, with the older one, “to a tittle,” which appeared almost a century earlier, and meant “to a dot,” as in “jot or tittle.” Beaumont used it in 1607, and it is probable that colloquial use long preceded his employment of the phrase.
Then as now Englishmen took pleasure in employing abbreviations and contractions, and we have no doubt that someone thought that “to a T” had a more amusing sound than “to a tittle,” and thus introduced our current expression.