Concertina wire or Dannert Wire is a type of barbed wire or razor wire that is formed in large coils which can be expanded like a concertina. In conjunction with plain barbed wire and steel pickets, it is used to form military wire obstacles.
In World War I, barbed wire obstacles were made by stretching lengths of barbed wire between stakes of wood or iron. At its simplest, such a barrier would resemble a fence as might be used for agricultural purposes. The double apron fence comprised a line of pickets with wires running diagonally down to points on the ground either side of the fence. Horizontal wires were attached to these diagonals.
More elaborate and formidable obstructions could be formed with multiple lines of stakes connected with wire running from side-to-side, back-to-front, and diagonally in every possible direction. Effective as these obstacles were, their construction took considerable time.
Barbed wire obstacles were vulnerable to being pushed about by artillery shells; in World War I, this frequently resulted in a mass of randomly entangled wires that could be even more daunting than a carefully constructed obstacle. Learning this lesson, World War I soldiers would deploy barbed wire in so-called concertinas that were relatively loose. Barbed wire concertinas could be pre-prepared in the trenches and then deployed in no-man’s-land relatively quickly under cover of darkness.
There was what might be called a concertina craze on: innumerable coils of barbed wire were converted into concertinas by the simple process of winding them round and round seven upright stakes in the ground; every new lap of wire was fastened to the one below it at every other stake by a twist of plain wire; the result, when you came to the end of a coil and lifted the whole up off the stakes was heavy ring of barbed wire that concertina‘d out into ten-yard lengths.
Concertina wire packs flat for ease of transport and 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 kilometer per hour. Such an obstacle is not very effective by itself, and concertinas are normally built up into more elaborate patterns as time permits.
Today, concertina wire is factory made and is available in forms that can be deployed very rapidly from the back of a vehicle or trailer.
Sources – Wikipedia