supplementary

supplementary — adjective

1. describing two angles that, when added together, total exactly 180 degrees and t

1.adjectiveB2
Definition

describing two angles that, when added together, total exactly 180 degrees and therefore form a straight line when placed side by side.

Example

If one angle measures 75 degrees, then its supplementary angle must be 105 degrees.

supplementary angle + measure [number] degrees

Quan found the supplementary angle for a 65-degree angle on the geometry worksheet.

Common collocations
  • supplementary angle
  • supplementary to
  • be supplementary (to)
Antonyms
  • complementary

    refers to angles totalling 90 degrees instead of 180

Grammar Patterns

supplementary + angle

supplementary to + noun phrase

Usage Note

In geometry, supplementary is often paired with complementary (angles totalling 90 degrees). A common exam task is to calculate a supplementary angle given one part of the pair.

Common Mistakes

Complementary angles add up to 180 degrees.
Supplementary angles add up to 180 degrees.
Complementary angles total 90 degrees; supplementary angles total 180 degrees.

2. offered or used as an addition to what already exists, so that the original thin

2.adjectiveB2
Definition

offered or used as an addition to what already exists, so that the original thing becomes better, more complete, or better supported.

Example

Christopher took a supplementary course in data analysis to strengthen his job skills.

supplementary course [in subject]

The doctor recommended a supplementary vitamin D tablet during the dark winter months.

supplementary vitamin / supplement

Common collocations
  • supplementary income
  • supplementary information
  • supplementary reading
  • supplementary materials
  • supplementary course
  • supplementary payment
  • supplementary budget
Synonyms
  • additional

    more neutral and widely used; lacks the nuance of 'filling a gap'

  • extra

    more informal; common in everyday speech

  • supplemental

    more common in American English; identical in meaning

  • auxiliary

    suggests a supporting or backup role, often for equipment or staff

Antonyms
  • main

    the primary source or item, rather than something added to it

  • primary

    the first or most important, not additional

Grammar Patterns

supplementary + noun

Usage Note

Almost always used before a noun — you would not say 'this information is supplementary' without following it with 'to' (e.g., 'supplementary to the main report'). In everyday speech, additional or extra are more common than supplementary.

Common Mistakes

We need a complementary report for the project.' (when meaning extra).
We need a supplementary report for the project.
Complementary means 'matching or completing each other well'; supplementary means 'added as an extra.'

Word family

Synonym discussion

Sense: adjective/2

All five words describe something added to an existing base, but differ in nuance and domain. Additional is the most neutral and widely usable — it simply means 'more on top of what exists.' Extra is the informal, everyday counterpart ('an extra slice of pizza'). Supplementary and supplemental both carry the idea of filling a gap or completing something that is lacking; supplemental is more common in American English, while supplementary dominates in British and academic writing. Auxiliary suggests a supporting or backup function, often for equipment, staff, or military forces. Complementary is the most distinct — it implies things that go well together or complete each other harmoniously ('complementary colours'), rather than simply adding quantity. For example, a supplementary textbook fills gaps in a course, while a complementary textbook pairs well with it stylistically.

additional ↗extra ↗supplemental ↗auxiliary ↗complementary ↗

Common collocations

Nouns

Prepositions

  • supplementary to

Etymology

LatinEnglish

The word comes from Latin supplementum, meaning 'something added to fill up a deficiency.' This was formed from supplere ('to fill up, make whole'), which itself breaks down into sub- ('up from below') and plere ('to fill'). It entered English in the early 1600s, first as a noun for something added to complete a document or text, then later developed the geometric sense in the 1800s.