I'm writing a technical paper, where I discuss the behavior of contacts between two slightly different materials. What is the correct way to write it:

  1. A contact between a green and a red balls

  2. A contact between green and red balls

  3. A contact between a green and a red ball

It sounds very simple, but I couldn't find a clear cut answer in any of my text books. I need to make it clear to the reader that I'm talking about a single contact between one red-type ball and one green-type ball.

Many thanks for your help!


'A contact between a green and a red ball' is both grammatically correct and clearly conveys that you are speaking of only one of each.

'A contact between green and red balls' is grammatically correct but allows for the possibility that there may be more than one of each.

'A contact between a green and a red balls' is just wrong.

You could also say 'between one green and one red ball'.

  • Many thanks for the detailed answer! Immediate follow up question: If I use a verb afterwards, is it correct to coordinate it as with plural, e.g. A red and a green ball are in contact ? – Leo Oct 24 '15 at 20:28
  • @Aleko: I'm not so sure about the second one you've listed. "between green and red balls" would inherently mean that multiple contacts occurred. "A contact" would then be incorrect. It would only apply if you group them, e.g. "A contact between a bag of green balls and a bag of red balls", since the bags are two single objects, much like in your first one where there is only one ball each. – Flater Jul 17 '17 at 14:19
  • @Leo: It often helps to substitute words for (grammatically) equivalent ones: "Joe and Bob are in contact" versus "Joe and Bob is in contact" should make it clearer. – Flater Jul 17 '17 at 14:21
  • @Leo Given that Aleko provided a thorough and well articulated answer, it might be nice if you up-voted it, and also gave consideration to designating it the "correct" response. – WS2 Sep 15 '17 at 22:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.