Using the LC connector involves nuances not present with other single-fiber connectors. First of all, the LC connector standard allows for two versions of the LC connector.
One version has a spring to apply force to the end-face of the ferrule. This is the senior LC.
The junior version of the LC connector does not have a spring.
The standard also requires that a mating of LC connectors must include at least one spring-loaded senior LC connector. This is required to ensure physical contact between connector end-faces. For this reason, LC connector adapters are offered in senior-senior versions or the more versatile senior-junior version. The presence of a key on the junior LC prevents the connector from being inserted into a senior adapter. However, the senior version without a key may be inserted into a junior or senior adapter. LC patch cords are made with senior LC connectors. This ensures that the matings at the interface between patch panel and electronics contain a spring-loaded LC senior connector.