A prefix is a letter or group of letters added to the beginning of a word. Knowing what a prefix means can help us to understand new words.

For example, if we add the prefix co- to a word, it means 'together'.

If a person is a co-author of a book it means they have written it with someone else.

We use a hyphen because without it the word coauthor would have three vowels together and would be difficult to read.

We don't always need to have a hyphen though, for example we can add co- to exist to make the word coexist without using a hyphen.

The prefix anti- means 'against'.

We use antiseptic cream to stop a wound from becoming septic (infected).

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