An incomplete grammar may be the result of attrition and fossilization of concepts in the L1 due to insufficient input once the child has switched to the dominant language.[7] Attrition is the loss of, or failure to make full use of, “grammatical knowledge previously acquired".[10] Fossilization occurs when a speaker lets go of non-core grammatical concepts but retains the basic core structure of the language.[7] Attrition and fossilization are considered to be part of the Incomplete Acquisition Theory because they render a language incomplete.