Bottleneck guitar is a variation or relation of slide guitar; instead of a cupping a metal slide in the fretting hand, the strings are fretted with a smooth, hard cylinder worn around a finger of the fretting hand, usually the fourth finger for a compromise between strength and freedom to fret chords with the other fingers, but sometimes the fifth (for more fingers free to fret chords) or the pointer (for enhanced strength). Like slide guitar, it gives a distinctive sound to both electric and acoustic instruments, with both enhanced clarity and volume (due to being fretted with a material harder than a fingertip), and the ability to 'slide' or 'glide' between notes, which is especially characteristic of blues.
The guitar is held in standard position, not flat on the lap as usual with slide guitar. As with slide guitar, the strings are usually of heavier gauge and higher tension than standard, as well as lifted further above the fingerboard.
This style of guitar originated in country blues, and, as the name suggests, with a broken-off bottleneck used as the fretting implement. Other items used for slides and noted in album notes or instructional books include other types of glass or metal tubing, a long 9/16 inch socket wrench socket, an old glass Coricidin pill bottle, and, in the case of Mississippi Fred McDowell "a beef bone, filed with a file". Each type of slide gives the sound a distinct characteristic in terms of tone, clarity and duration.
The guitar is usually tuned in an open tuning, where the strings are tuned to sound a chord when not fretted; thus sliding the bottleneck up down the neck, parallel to the frets, moves the same chord up the scale. Occasionally a bottleneck is used on only the highest two strings of a guitar in standard tuning, usually in live performance to introduce just a short passage of bottleneck effect into a piece which otherwise consists mainly of guitar played in standard fashion. In this case, the bottleneck is usually worn on the fifth finger, leaving the others free to fret chords in normal fashion.