There are two separate knowledge systems underlying second language performance, the acquired system, which consists of subconscious knowledge of the second language grammar, like the subconscious knowledge native speakers have of their first language, and the learned system, the product of formal instruction (typically classroom language teaching), comprising conscious knowledge of “easy” second language grammatical rules.

The acquired system is typically the only knowledge source speakers can use in real-time communication, when they are attending to meaning, not form; the learned system serves only as a planner and editor with which to inspect, or monitor, the output of the acquired system.

Although stemming from different sources, the acquired system and the learned system are interconnected, interfacing with one another, which means, for instance, that the acquired system can be developed by the learned system through formal instruction, and the learned system can, under appropriate circumstances, be made faster through contact with native speakers.