Black's Law Dictionary (2nd edition)dictionaries

Four Corners

The face of a writ-ten Instrument. That which is contained on the face of a deed (without auy aid from the knowledge of the circumstances under which it is made) is said to be within its four corners, because every deed is still supposed to he written on one entire skin, and so to have but four corners

Source: Black’s Law Dictionary 2nd Ed (1910)