How to search for two groups of field values?

[arrowecssupport]

So I've got 2 different values I'm trying to use; letters & numbers. I want to be able to say

If letters = a b or c & numbers = 1 2 or 3

index = test letters = "a" OR letters = "b" OR letters = "c" AND numbers = "1" or numbers = "2" OR numbers = "3"

I don't think this is quite right. Any ideas?

[woodcock]

You need parentheses:

 index = test (letters = "a" OR letters = "b" OR letters = "c") AND (numbers = "1" or numbers = "2" OR numbers = "3")