The octave mandolin is a fretted string instrument with four pairs of strings tuned in fifths, G, D, A, E (low to high), an octave below a mandolin. It has a 20 to 23-inch scale length and its construction is similar to other instruments in the mandolin family. Usually the courses are all unison pairs but the lower two may sometimes be strung as octave pairs with the higher-pitched octave string on top so that it is hit before the thicker lower-pitched string. Alternate tunings of G, D, A, D and A, D, A, D are often employed by Celtic musicians.