What is the difference between "parentheses" and "parenthesis"?

  • 2
    As the answers point out, the one ending in -is is the singular, and the one ending in -es is the plural. They're pronounced differently, too. The singular in -is is pronounced /pə'rɛnθəsəs/, and the plural in -es is pronounced /pə'rɛnθəsiz/; the last syllable is identical to sieze or seas. The difference in pronunciation is how we really tell; the spelling is Greek and doesn't represent English well at all. And, as you demonstrate, it's easy to confuse. Commented Oct 6, 2013 at 15:29

3 Answers 3


Parenthesis is of course Greek and means in fact "insertion". It has taken the meaning of the signs " ( " or " ) ".

Parentheses is the regular plural.

Usually, you use a pair of the signs showing an insertion, then "between parentheses" - or brackets ; however, "in parenthesis" means : as an afterthought.


If you're still confused after the different emphases given by Mark and Barrie,

1 A parenthesis is

  • one of the marks usually called (ordinary, or curved) brackets in the UK


  • material set off by a pair of brackets (or commas or dashes).

I've just used four parentheses to enclose two parentheses – but I wouldn't usually mix senses like this as it gets too confusing.

Actually, since one bracket is rarely found without its 'other half', the singular form parenthesis more often refers to the inserted material, but this is not mandatory. To really disambiguate, the term 'parenthetical', used as a noun, is often used for the 'inserted material' sense.

2 Parentheses is usually used to refer the pair of brackets often used to set off a parenthetical – but the term could also be used for more than one parenthetical.

See http://josecarillo.blogspot.co.uk/2009/12/parenthesis-isnt-just-optional-material.html for further thoughts on the matter.

  • 1
    At first glance, this would seem confusing, but it actually does throw a lot of light on the matter. Thank you! Commented Jun 10, 2020 at 17:23

‘The Cambridge Guide to English Usage’ describes parenthesis as ‘a string of words interpolated into a sentence but syntactically independent of it’.

It describes parentheses as:

. . . the standard name for round brackets in the US and Canada, and increasingly in Australia. In the UK parentheses is still mostly a technical term, by BNC evidence, where it appears in scientific and bureaucratic documents. But overall British writers are about three times more likely to use in brackets than in parentheses.

This means that if I write a sentence (I don’t mean with a pen) the words ‘I don’t mean with a pen’ are a parenthesis, and the marks at the beginning and end of them are parentheses.

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