Could anyone help me to clarify the difference between ', or' and 'or'? Sometimes I see articles are intentionally written this way rather than just reflex. I've made examples below, and explained by myself.

'Or' case

  1. That system is the circulatory system, or the cardiovascular system

→ In this case, ', or' suggests that both 'the circulatory system' and 'cardiovascular system' are the same thing, but have different names

  1. That system is the circulatory system or the cardiovascular system

→ In this case, 'or' suggests that 'the circulatory system' and 'cardiovascular system' are two different things, and that 'that system' I am talking about can be either of 'cardiovascular' or 'circulatory'.

  • A "coma" is halfway between asleep and dead. A "comma" is a punctuation mark. The two words are also pronounced differently. – Jasper Jan 11 at 3:32
  • In most cases where a comma is needed, there should be a space after the comma, but not before the comma. – Jasper Jan 11 at 3:34
  • Thank you Jasper. The reason why there is such space is because I cannot make it 'bold' without leaving some space. Thank you so much for helping me to recognise my mistake associated with 'comma' – PandoraU.U.D Jan 11 at 3:46
  • 1
    @PandoraU.U.D Note that you can get around the issue of spaces and formatting if you use HTML code in your post. For example<b>, the comma and this</b> is bold even though there is no space before the comma. (However, that trick only works in posts, not in comments.) – Jason Bassford Jan 11 at 9:01

There are two cases you need to use the comma before "or". The first is in your example, to set apart a clause. In the first example "or the cardiovascular system." is an appositive clause, renaming the circulatory system.

The second case you should be aware of is the oxford comma in a list of three or more things. "We want apples, bananas, or oranges." There is some debate over the necessity of this (and I am firmly in the "necessary" camp).

However, if you're separating two things that are comparative, as in your second example (if the circulatory system and the cardiovascular system were two different things), you don't put a comma before the "or". "We want apples or oranges." You will often see this combined with "either". "That system is either the circulatory system or the cardiovascular system." "Actually, Joe, they're the same thing."

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