Old connector guys will tell stories of the days when connectors were described as having "primary" and "secondary" alignment" mechanisms. The primary mechanism was usually taken to be the alignment of the optical fibers provided by the outside surface of the connector feedthrough (the device allowing two connectors to be mated). Secondary alignment is that internally provided by fixtures within the feedthrough. The more alignment features a connector style feedthrough exhibits, the lower  will be the mated connector’s optical attenuation. For example, in the older 905-type subminiature assembly connector (SMA), there is only the primary alignment mechanism offered by the feedthrough. But for the 906-type SMA connector, the small "shoulder" machined onto the 906-connector ferrule offered both a primary and secondary alignment. When a small plastic "full" sleeve is placed over the shoulder of the 906 ferrule, and the assembly is inserted into the feedthrough, it allows a mating 906-connector ferrule to slide into the other side of the full sleeve. This also internally aligns the ferrules. In addition, the outer surface of the feedthrough still provides a primary alignment mechanism. This is the reason why the 906 style SMA was more expensive and had a lower insertion loss than its 905 predecessor. With the advent of the ceramic-tip SMA, both styles are still available.

With the popularity of the ST connector which has both metallic and ceramic ferrules, do the primary and secondary mechanisms still exist? The answer is yes. Inside of the ST feedthrough is a permanent sleeve which no longer has to be separately attached as with the SMA connector. This ST sleeve can be either metal or ceramic material. The outer housing of the ST feedthrough still provides a primary alignment scheme, and the inside sleeve provides the secondary alignment method.

A mistake that is often made mismatches the connector ferrule material and the feedthrough sleeve material. In other words, some designers choose a metal sleeve feedthrough for use with a ceramic ST ferrule, or the opposite. Since the ceramic will eventually wear down the metal, the mating and unmating process if done enough times, will allow the secondary alignment mechanism to become loose. This will result in a radial mismatch of the fibers producing a higher than expected insertion loss for the mated fibers. To avoid problems, be sure to match the feedthrough material to the connector ferrule material in any design.