The shape is not always that of a symmetrical figure-of-eight, because a "bite" is often taken out of the neck to facilitate the left hand reaching the furthest frets, which are located on the part of the neck that is attached to the sound box. A so-called pick-guard is often found under this part, which is used to protect the top of the sound box against damage from the plectrum with which the rhythm guitar is always played. Instead of a sound hole there are often two f shaped openings on the top. The guitar is strung with twelve steel strings in six pairs of two, stretched over a bridge that produces additional vibration.
The dobro is a form of rhythm guitar with an unusual shape that was very popular with the early blues singers. Its bridge was mounted on a round metal resonator and the strings were played on the neck with one finger of the left hand on which a kind of thimble was worn. The sliding of the finger over the strings caused a very special effect.
The rhythm guitar is mainly played in chords and accompanies vocals. It provides the chord sequence or "progression" of a composition, frequently providing the rhythm or "beat" as well, usually as part of a rhythm section. Rhythm guitarists may employ an electric acoustic guitar or a humbucker-equipped electric guitar for a richer and fatter output. Also, rhythm guitarists may use strings of a larger gauge than those used by lead guitarists. Fingerings have been developed for the rhythm guitar, so even those who can't read music can learn to play it. In ensembles or "bands" playing within the acoustic, country, blues, rock or metal genres (among others), a guitarist playing the rhythm part of a composition supports the melodic lines and solos played on the lead instrument or instruments, be they string, brass, wind, keyboard or even percussion instruments, or simply the human voice.