The story of the Nail reaches back to the early history of weights and measures and is of philological as well as metrological interest. The half-cubit or span, the common handy measure in most parts of the world, is of 12 digits, while the foot is 16 digits and is still so divided in Italy and other southern countries. The digit is not only a middle-finger breadth, it is also a thumb-nail breadth; as the former it was in Greek _dactylos_, as the latter _onyx_, which became _onkia_ in Southern Italy and gave rise to two Latin words, _unguis_ for the actual finger-nail, _uncia_ for the thumb-nail breadth equal to the digit and generally for a twelfth part. Hence a differentiation of meaning in the Romance languages. GREEK _onyx_, _onkia_ Latin _unguis_ _uncia_, thumb-nail breadth, ounce Italian _unghia_ _oncia_, last thumb-joint, ounce Provençal _ounglo_ _ounço_, finger-joint, knuckle, ounce French _ongle_ _once_, finger-joint (obs.), ounce English (nail) _unce_, ynch When the Romans adopted the duodecimal or ‘uncial’ system they applied it to the foot, which was divided into either 12 or 16 parts both called unciæ; but to distinguish these they used two other words, _digitus_ for the sixteenth and _pollex_, thumb, for the twelfth, the thumb-breadth. In English ‘unce, ynch’ always meant the thumb-breadth 1/12 of a foot, ‘Nail,’ the thumb-nail breadth equal to the digit, being kept for the 1/16 foot. Thence ‘nail’ came to have the general sense of sixteenth and to be applied to that fraction of a 4-span yard, of a 5-span ell, of a bushel, of a hundredweight. In Latin the analogous general sense of twelfth belonged to _uncia_, whether of the foot, of the land-unit, of the pound. The general sense of twenty-fourth attached to the scruple as 1/24 ounce, passed to the qirát, or carat, in the countries influenced by Arab customs, as being 1/24 of the mithkal, the Arab successor of the Roman solidus. In modern Italy the palmo or span, and the libbra or pound, were both divided into 12 _oncie_, meaning inches or ounces. With the general substitution of the 16-ounce pound for that of 12 ounces, the word ‘ounce’ lost its meaning of twelfth. In some of the Romance languages its sense of length extended to the length of any finger-joint, especially to the length of the proximal joint of the thumb. Thus in Southern France the _ounço dóu pouce_ (Fr. _once de poulce_) was taken as 1/5 of the span or nearly 2 inches. When our Cwt. was raised to 112 lb. and the 16-lb. stone replaced by that of 14 lb. the term Nail was applied to the half of the new stone, and it was perhaps the divisibility of the new Cwt. into 16 parts of 7 lb. that reconciled people to the unpopular new weight. But for all that, the people held on for centuries to the 16-lb. stone, and call its half, 8 lb., a nail, though it is no longer the sixteenth of a larger unit.