English Reference
adjective · noun

complementary

adj. · n.
US /ˌkɑmpɫəˈmɛntɹi/ UK /kˌɒmplɪmˈɛntəɹi/ com·ple·men·tary UK spelling complementaries

  1. 01 B2 Upper Intermediate

    adj.describing two things that work well together because they each provide what the other is missing.

    adj.acting as a complement; completing a whole or enhancing the qualities of another through contrast or addition.

    Simple

    The two sisters have very different but complementary personalities.

    In context

    The merger was successful because the two companies possessed complementary strengths in manufacturing and digital marketing.

    Usage

    Often used with the preposition 'to'.

    Pitfall
    The manager gave me a complementary ticket. The manager gave me a complimentary ticket.

    Use 'complementary' for things that complete each other. Use 'complimentary' for things that are free or involve praise.

  2. 02 C1 Advanced Technical Science

    adj.describing how parts of DNA or RNA fit together perfectly like a puzzle.

    adj.relating to the specific pairing of nitrogenous bases in DNA and RNA molecules.

    Simple

    In DNA, the base adenine is always complementary to thymine.

    In context

    The double-helix structure relies on complementary base pairing to ensure that genetic information is copied accurately during replication.

  3. 03 B2 Upper Intermediate

    n.one of two colours that look very different but look good together, like red and green.

    n.one of a pair of primary or secondary colours that, when combined, create a neutral colour like grey or white.

    Simple

    She chose orange as a complementary to the blue walls of the room.

    In context

    The artist used a vibrant purple as a complementary to the yellow highlights, creating a sense of visual tension in the portrait.

  4. 04 C1 Advanced Technical Math

    n.an angle that, when added to another angle, makes exactly 90 degrees.

    n.an angle that adds with another to equal 90 degrees or a right angle.

    Simple

    If one angle is 30 degrees, its complementary is 60 degrees.

    In context

    The architect calculated the complementary of the roof's pitch to ensure the support beams met at a perfect right angle.

Don't confuse

complementary vs complimentary — Complementary describes things that complete each other, while complimentary describes praise or items provided for free.

Usage complementary approach

We see it as a protection complementary to the storm shelter,” Houri says.

Ars Technica· 13 Aug 2026

Dr Yasir Ali, a reader in Transport Modelling at Loughborough University, says e-scooters are "complementary" to other modes of transport, rather than replacing them.

BBC News· 22 Jul 2026

From complement + -ary. Piecewise doublet of complimentary.

Etymology adapted from Wiktionary, available under CC BY-SA 4.0.

This entry
Level B2 · Upper Intermediate
Frequency Frequent
Updated