Guitars usually have 6 strings, although there are variations on this, such as the 12-string, 7-string, 4-string (ukulele) and the bass guitar, which usually has 4 but also exists in 5, 6, 8 and 12.
The weight of a string is determined by its diameter and is normally measured in thousandths of an inch. The larger the diameter the heavier the string is (with thinner strings being lighter).
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Heavier strings require more tension for the same pitch and are consequently harder to hold on to the fretboard. Heavier strings will also produce a louder note and for this reason steel-strung acoustic guitars will normally be strung heavier than electric guitars. On electric guitars, heavier strings may also produce a thicker tone, leading to their use by rhythm guitarists in rock music.