Finger (unit)
A finger (sometimes fingerbreadth or finger's breadth) is any of several units of measurement that are approximately the width of an adult human finger. [Exactly which part of the finger should be used is not defined; the width at the base of fingernail (#6 in the sketch) is typically less than that at the knuckle (#5).]
The digit, also known as digitus or digitus transversus (Latin), dactyl (Greek) or dactylus, or finger's breadth – 3⁄4 of an inch or 1⁄16 of a foot. (about 2 cm)
In medicine and related disciplines (anatomy, radiology, etc.) the fingerbreadth (literally the width of a finger) is an informal but widely used unit of measure.
In the measurement of distilled spirits, a finger of whiskey refers to the amount of whiskey that would fill a glass to the level of one finger wrapped around the glass at the bottom.
Another definition (from Noah Webster): "nearly an inch."
Finger is also the name of a longer unit of length, used historically in cloth measurement, to mean one eighth of a yard or 41/2 inches. (114.3 mm) Again, which finger and whose finger, is not defined.
These units have no legal status but remain in use for 'rough and ready' comparisons.