The twins look alike—so much so that even their parents sometimes confuse them.
What is 'that' and what is its function?
The twins look alike—so much so that even their parents sometimes confuse them.
What is 'that' and what is its function?
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.)