MPO connector usually has 12 fibers. 24 fibers, 36 fibers and 72 fibers are also available.
Each MTP connector has a key on one of the flat side added by the body. When the key sits on the bottom, this is called key down.
When the key sits on top, this is referred to as the key up position. In this orientation, each of the fiber holes in the connector is numbered in sequence from left to right and is referred as fiber position, or P1, P2, etc.
A white dot is additionally marked on one side of the connector to denote where the position 1 is. (shown in the following picture) The orientation of this key also determines the MPO cable polarity.
