Concertina wire is a type of barbed wire or razor wire that is formed in large coils which can be expanded like a concertina. Each coil actually consists of two oppositely wound helices which support each other against crushing whilst allowing easy longitudinal movement. It is used to form military wire obstacles.
Concertina wire packs flat for ease of transport, but can then be deployed as an obstacle much more quickly than ordinary barbed wire. A platoon of soldiers can deploy a single concertina fence at a rate of about a kilometre per hour. Such an obstacle is not very effective by itself, and concertinas are normally built up into more elaborate patterns as time permits.
See also: abatis, wire entanglement