Query: why "salt and pepper" but not "pepper and salt"?

[Douglas G. Wilson]

>>This evening I received the following query: Why do we always say
>>"salt and pepper" and never "pepper and salt?"

>I believe it's one of the "freezes" or fixed binomials listed in
>Cooper & Ross's "World Order" paper on the topic in the CLS
>Functionalism volume (1975).

But the order is not fixed in all contexts, surely. MW3 defines
"salt-and-pepper" simply as "pepper-and-salt", under which is given the
usual meanings along the lines of "flecked dark and light". There are also
entries for "pepper-and-salt cat" and "... moth". In my experience
"pepper-and-salt mustache" for example is more frequent than
"salt-and-pepper ...".

As for "please pass the ..." etc. I've heard "pepper and salt" although
less often than "salt and pepper". However, presumably the combination of
condiments is ancestral to the color expression, so I suppose "pepper and
salt" cannot have been too absurd in the popular mind even in the condiment
sense.

It is my casual impression that the order here is less fixed than in some
other combinations such as "bread and butter".

-- Doug Wilson