Serving to complete or supplement something; adding what is lacking to make something whole.
From complement (Latin complementum, 'that which fills up') plus the adjective suffix -al. This technical term developed in the 16th-17th centuries, particularly in scientific and philosophical contexts.
Complemental is rarer than 'complementary,' but they mean slightly different things: complemental means 'serving to complete,' while complementary means 'forming a harmonious pair'—a subtle distinction few people notice.
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