The twins look alike—so much so that even their parents sometimes confuse them.

What is 'that' and what is its function?

  • 1
    "That" belongs to the category (POS) subordinator and its function is that of marker. Here "that" is introducing the declarative content clause "that even their parents sometimes confuse them" functioning as complement.
    – BillJ
    Feb 22 at 16:52
  • I stand to be corrected, but I think [that] when "that" is included as a subordinator (conjunction introducing a subordinate clause / complement), it's always optional. It certainly is in the examples here (both OP's and mine). Feb 22 at 18:12
  • @FumbleFingers In case of "the fact/idea/belief, etc. that S+V, the 'that' is not optional.
    – gomadeng
    Feb 22 at 18:36
  • The fact I'm a native speaker cuts no ice with you, then! I'd be very surprised if you've actually read any "authoritative source" supporting your claim, so I'm inclined to suspect you're making it simply because you know English well enough to be aware that that specific context is less likely to discard "optional that" than most! Feb 22 at 19:07
  • @FumbleFingers It's a basic one in my country. The oppositive 'that' is a must and the 'that' is classified as a noun clause due to the fact that the antecedent is a noun: the belief, the fact... .
    – gomadeng
    Feb 22 at 19:31

1 Answer 1


According to Oxford Languages it's a conjunction introducing a subordinate clause expressing a result. (The result of their being 'so alike' is that their parents sometimes confuse them.)

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