The question Precedence of “and” and “or” asks if there is any notion of precedence ordering in the English and it would seem not, based on the answers.

Regardless of that, if you saw the following piece of text, how would interpret it and why?

The prerequisite(s) of module AB1234:

GN3001 and GN3002 or BC3006 and BC3007.

  • 2
    (GN3001 and GN3002) or (BC3006 and BC3007)
    – Em1
    Feb 20, 2012 at 11:27
  • @Em1 What if the department codes were all different? Would you then change the axes to the ands?
    – Adam Lynch
    Feb 20, 2012 at 11:40
  • 1
    I would NOT interpret -- it carries with it the unwritten rule "if in doubt ask, do not guess". The English language, as you correctly stated, does not provide precedence rules.
    – Kris
    Feb 20, 2012 at 11:41
  • @AdamLynch I don't consider the codes. Take notice of my answer below.
    – Em1
    Feb 20, 2012 at 13:16
  • I'd interpret it as evidence that these are not the people I want to study with, if that's their best effort at unambiguous communication. What if the courses themselves were equally vague? I'd spend all my time asking for clarification. Better to go somewhere else in the first place! Feb 21, 2012 at 0:58

5 Answers 5


I would intepret it as

either "GN3001 and GN3002"

or "BC3006 and BC3007"

are required

But I would also consider that it is badly formulated, and they should have made it crystal clear.

One reason for interpreting this in this way is that the two pair would appear to go together - two GN units or two BC units makes sense ( without knowing what they are ). So there are subtle suggestions in the content that would point me one way or the other.

However, when this is being defined as course pre-requisites, it should be made completely clear and unambiguous. I would feel a need to contact whoever to clarify this, which means that the communication has failed.

  • Ah didn't even notice the GN/BC split.
    – Adam Lynch
    Feb 20, 2012 at 11:35
  • Plus I love your username. Schroedinger didn't mention broadband was also available within the box. 50/50 chance it works, right?
    – Adam Lynch
    Feb 20, 2012 at 11:36
  • The cat has to have something to do while not being dead.... Feb 20, 2012 at 12:37

We read and interpret one word at a time so, when sentences are ambiguous, we tend to take the narrowest left-hand interpretation. That's why we find garden path sentences (e.g. “Fat people eat accumulates.” or “I convinced her children are noisy.”) so confusing.

In the case of your sentence, when we reach this point:

The prerequisite(s) of module AB1234:

GN3001 and GN3002 …

it looks like GN3001 and GN3002 forms a group so, until we see evidence to the contrary, we keep going with that interpretation. This means that, on first glance, most people would interpret the sentence as meaning:

(GN3001 and GN4003) or (BC3006 and BC3007)

rather than:

GN3001 and (GN4003 or BC3006) and BC3007

  • Why not ((GN3001 and GN4003) or BC3006) and BC3007) OR AA1001 and (BB2002 or (CC3003 and DD4004)) if we're going from left to right?
    – Adam Lynch
    Feb 20, 2012 at 23:38
  • @Adam Lynch We jump to conclusions early. (Notice that you can still understand what I'm saying even if I don't give the last word in the _______.) Consequently, we tend to group things to the left. This usually works -- if you read "A and B and C" you can group it as "(A and B) and C" without any problem -- but garden path sentences show that there are limitations.
    – Pitarou
    Feb 21, 2012 at 1:44

Being a fairly pedantic person, I would interpret it only as being highly ambiguous, and I would ask the author for clarification.

  • haha - you're obviously more tolerant than me! I'd just rule this one out of my "places I might want to study at", and look for somewhere they have a better grasp of the basics of communication. Feb 21, 2012 at 1:01

The natural way to read it is as SchroedingersCat already mentioned. But this does also works if the pairs were mixed:

(GN3001 and BC3006) or (GN3002 and BC3007)

It has nothing to do with similar IDs for units, it's just a question of how the man's brains works. It isn't important what the numbers are, another example

AA1001 and BB2002 or CC3003 and DD4004

is also read like the first example above:

(AA1001 and BB2002) or (CC3003 and DD4004)

This construction is the simplest way how to structure them and thus this is the way our brain/mind works.

Think of the alternatives:

  1. AA1001 and (BB2002 or CC3003) and DD4004
  2. AA1001 and (BB2002 or (CC3003 and DD4004))

Even with parenthesis it's not that easy to read. The first alternative interpretation is unlikely, because it does absolutely not fit our brain's/mind's structure. If you wanted to intend that one you ever have to clarify it.

The latter would only concur with our mind structure if you know that the value, performance, ... of BB2002 is greater than or equal to CC3003 and DD4004 together. Though, it is only secondary in our mind, because the logic behind it is also much more complicated.

Thus, in a nutshell, you first connect the both "and"-condition and then choose either of them as the "or"-condition requires.


There is some sense in saying that the precedence of "and" is greater than that of "or". Any given proposition is either true or false (or weird, like "This statement is false," but we'll ignore those here). From a proposition you can obtain more complex propositions using various logical operations, such as "and", "or", and "not". For a more complex proposition built out of simpler propositions and logical operations, you can figure out for any set of truth values for the simpler propositions whether the more complex proposition is true or false. You can represent all truth values of all of the simple propositions and of the more complex proposition using a truth table. If you use 0 to represent false and all other numbers to represent true, multiplication is the same operation as "and" and addition is the same as "or". The standard order of operations gives multiplication higher precedence than addition, which would translate into "and" having higher precedence than "or". However, this is only by convention and thus there is strictly no "from first principles" reason to say that "and" has higher precedence than "or".

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