Twin loop binding wires (also called duo-wires or double loop wires) are offered in two pitch patterns – 2:1 and 3:1. The 2:1 pattern has two rectangular holes per inch, with each letter-sized sheet containing a total of 21 holes. The 3:1 pattern is made up of three square holes per inch, and each 11″ sheet having 32 holes. Unless your wire binding machine is dual-function or modular (which would allow you to change out your punching dies), it will only punch one of the two pitches.
3:1 pitch is designed for smaller documents, with wires ranging in size from 1/4″ to 9/16″. The 3:1 pitch wires will accommodate booklets 20-120 pages thick (or 3/16″ to 1/2″ thick). The 2:1 pitch pattern is reserved for larger documents totaling between 110 and 260 pages (or 9/16″ to 1-1/8″ thick). Twin loop wires that have a 2:1 pitch are available in sizes from 5/8″ to 1-1/4″.