A theorbo, also sometimes called a chitarrone, is a lute with a long neck extension. A theorbo has two pegboxes, one at the top of the fingerboard and the other at the end of the extension. The extended neck is necessary allows for a clear and sustained sound from low bass strings.
The theorbo was designed to produce a large volume of sound, specifically for accompanying singers or other instrumentalists. On historical instruments, the fretted strings running to the first pegbox are generally between about 70 and 90 cm long, and the extended basses may be from 150 to almost 180 cm in length.
The instrument pictured is single strung throughout – each of the 14 strings stands alone. Some theorbos have double stringing for the courses that are fretted, but the extended basses are almost always single. A theorbo uses a re-entrant tuning scheme, with the top two strings tuned an octave lower than might be expected, as shown below.
More detailed information on the theorbo is available on a site maintained by Lynda Sayce.