The short answer
The length of a glove up the wrist and arm is traditionally measured not from the fingertips but from the thumb seam at the base of the thumb. The traditional unit of measurement is a 'bouton' or, in English, a 'button'. At Cornelia James, we measure our button length in inches, so a 6 button length glove comes up the arm six inches from the seam at the base of the thumb.
The long answer
The (largely French) master glovemakers who dominated the industry in medieval times were accustomed to placing a button roughly every inch up the sleeve of a long glove. They measured the interval between buttons using the length of their thumb from knuckle to thumb tip. This measurement is known, in French, as a 'pouce', which translates into English as both 'thumb' and 'inch'. Confused? We don't blame you! The 'button' is somewhat lacking in precision but one must allow for the fact that the 'button' emerged as a unit of measurement when land was measured in 'rods' - the aggregate length of the left feet of 16 men lined up heel to toe as they emerged from church - and furlongs - the length of the furrow that a team of oxen could plough without stopping to rest.
At Cornelia James, the Master Glovemaker has a thumb which measures 1.33 inches, give or take, from knuckle to tip, so that our classic opera length glove would measure 16 inches up the arm from the thumb seam or just over 12 buttons. Roughly.
Even in the Middle Ages, the lack of an international standard was problematic and the cause of much strife and general aggravation. Today we face a global market and, moreover, we worry that, if we were to lose our Master Glovemaker and have to replace him, we would have to recalibrate our entire collection with reference to a different sized thumb. We like tradition as much as anyone and will cling to the use of the term 'button' to denominate glove length but, in the interests of clarity and accuracy in our globalised world, we have determined that our 'button' shall be equal to an inch. In metric terms that's precisely 2.54 cm. Or roughly a pouce.