To a T

To a T meaning exactness or precision dates from the late 17th century but the origin is uncertain. Some sources maintain that it derives from the measuring tool, the T-square, but this was not known as such until about one hundred years later c. 1785. A more plausible theory, and one supported by the OED, is that it could be an abbreviation of ‘to a tittle’, tittle being the technical name for the horizontal cross stroke on the lower case letter t, which dates from the 1400s. If this theory is correct, then it could also be the source of the early 19th century expression, dot the i’s and cross the t’s because the dot on the lower case letter i is also known as a tittle.