Mandolin are used. Usually, courses of 2 adjacent strings are tuned in unison. By far the most common tuning is the same as violin tuning, in scientific pitch notation G–D–A–E, or in Helmholtz pitch notation: g–d′–a′–e″. Note that the numbers of Hz shown above assume a 440 Hz A, standard in most parts of the western world. Some players use an A up to 10 Hz above or below a 440, mainly outside the United States. Other tunings exist, including "cross-tunings", in which the usually doubled string runs are tuned to different pitches. Additionally, guitarists may sometimes tune a mandolin to
String (structure) playability. The invention of wound strings, such as nylon covered in wound metal, was a crucial step in string instrument technology, because a metal-wound string can produce a lower pitch than a catgut string of similar thickness. This enabled stringed instruments to be made with less thick bass strings. On string instruments that the player plucks or bows directly (e.g., double bass), this enabled instrument makers to use thinner strings for the lowest-pitched strings, which made the lower-pitch strings easier to play. On stringed instruments in which the player presses a keyboard, causing a mechanism to strike the strings, such