 Machine
heads are designed to adjust the
tension of the strings, raising or lowering
their pitch to tune the instrument.
While machine heads are typically screwed
into the headstock and designed with
gears to allow finer tuning, some simpler
designs simply involve wooden pegs held
in place by friction.
There are three types of machine head
on modern steel-strung guitars:
* Open back (where the gear itself
is exposed);
* Closed back (where the gear is covered
and the occasional lubrication of the
gear can be done through a small hole
in the cover);
* Sealed (closed back, but permanently
and completely sealed and lubricated,
eliminating any maintenance).
 These
may be arranged on the headstock in
two rows of three, as on the Gibson
Les Paul and the majority of acoustic
guitars, or six in line, as on the Fender
Stratocaster. Another arrangement, notably
less popular, is one row of four on
the playing side and one row of two
on the opposite side. Music Man guitars
feature this type of setup.
 Nearly
all modern nylon-strung (or classical)
guitars use open geared machine heads.
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