know from

[Mark A Mandel]

On Fri, 27 Dec 2002, Philip Trauring wrote:

#I think there are two different constructions here. One is 'know
#from' and the  other is 'know X from Y'.

I agree.

#Examples such as 'he doesn't know his ass from his elbow' are also
#idioms, whose constructions I would argue are not easily tracked and
#frequently flaunt normal grammatical rules. Does anyone have an
#example of this 'know X from Y' construction that isn't an insult?

This is made-up but imho plausible, and deprecatory but not just an
insult, but rather contentful:

        You can't hire her as our webmaster! Why, she doesn't
        know Java from Javascript!

(Neither do I, but I'm not applying for a job that requires it.)

So I don't think 'know X from Y' is an idiom. It's a construction
meaning 'know / recognize / understand the difference between X and Y'.
"Doesn't know shit from shinola" / "his ass from a hole in the ground"
are idioms built on literal, grammatically correct uses of the
construction.

"Know from X" is a different breed of equine entirely.

-- Mark A. Mandel