The missing-letter effect for common function words depends on their linguistic function in the phrase.

It has been proposed that function words such as for and on conceal their letters because their higher familiarity allows fast access to their unitized representations. However, in this study we show that letter detection in function words varies with their linguistic role in text. When such words were embedded in a phrase where they were forced into a content role by the surrounding context (e.g., for or against or on switch), letter detection improved markedly and did not differ from that of matched content words. The result was replicated when the context preceding the function word and the overall sentential meaning were equated for both function and content usages