The qin is a very quiet instrument, with a range of about four octaves, and its open strings are tuned in the bass register (its lowest pitch is about two octaves below middle C, or the lowest note on the cello). Sounds are produced by plucking open strings, stopped strings, and harmonics. Stopped sounds are special for the variety of slides and ornaments used, and the use of glissando (sliding tones) gives it a sound similar to a pizzicato cello or fretless bass guitar. Extended sections in music scores consisting entirely of harmonics are common, this made possible because the 91 indicated harmonic positions allow great flexibility; early tablature shows that even more harmonic positions were used in the past. By tradition the qin originally had five strings, but ancient qin-like instruments with 10 or more strings have been found. The modern form has been standardized from about two thousand years.