There are three types of jack configurations that are commonly encountered in audio systems--pairs of jacks that are either full-normaled or half-normaled, and single (or non-normaled) jacks.
A jack used for normaling has extra sets of contacts that are used to connect it to a similar jack (usually an upper jack to a lower one on a two-row jackfield) plus switch contacts that open when a patchcord is inserted into the jack.
With a full-normal pair, the normal connection is broken when a patchcord is inserted in either the upper or lower jack. This means that whatever source was present on the upper jack will no longer be automatically connected (normaled) to the destination on the lower jack.
For a half-normal, the normal connection is broken only when a patchcord is inserted into the lower jack. When a patchcord is inserted into the upper jack, it is multed (connected together) with the source already connected to that jack. The source signal continues to be normaled to the destination device.
Single jacks used for such signals as inputs and outputs of utility processing devices, tie lines, and distribution amp monitor outputs, etc.; signals that don't need to be normaled to anything. The signal appears on the jack and is available for patching to or from another device.
