Concertina wire is a type of barbed wire or razor wire that is formed in large coils which can be expanded like a concertina, a small hand-held bellows-type instrument in the same family as the accordion. In conjunction with plain barbed wire and steel pickets, it is most often used to form military-style wire obstacles , although these are not necessarily always used in a military setting, such as when used in prison barriers, detention camps or riot control.