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I've learned that commas must be put before and, but, so, ect. However, I came across this sentence in a reading test.

Exercise causes blood vessels to open up so blood flows more easily.

Why is there no comma before so? Must I put it there?

  • No comma is used before "so" (or "so that") when the linker indicates purpose. A comma is used before "so" (or "so that") when the linker indicates consequence or result. – Gustavson Sep 12 '17 at 2:15
  • Is 'blood flows more easily' a result of 'exercise causes blood vessels to open up'? – Thanhgiang Sep 12 '17 at 2:18
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The comma in question separates coordinate clauses. 

It's not the conjunctions themselves that require this comma.  It's the compound sentence structure that requires both a comma and a coordinating conjunction.  The words "and", "but" and "or" function only as coordinating conjunctions, so compound sentences joined by them will always* include the comma. 

The words "so", "for" and "yet" sometimes function as coordinating conjunctions, but they also have other functions.  "So" is an interesting word because it sometimes functions as a subordinating conjunction. 

Your example sentence uses "so" as a subordinating conjunction. 

 

With the comma, we have a compound sentence: 

Exercise causes blood vessels to open up, so blood flows more easily. 

Here, the two clauses are independent.  Each clause could stand as a separate sentence. 

 

Without the comma, we have a complex sentence: 

Exercise causes blood vessels to open up so blood flows more easily. 

Here, "blood flows more easily" is subordinate.  The phrase "so blood flows more easily" modifies the phrase "to open up". 

 

There is practically no difference in meaning between the complex and compound versions of that sentence.  The difference might be easier to see given a different example: 

He's throwing a party so I can meet her. 
He's throwing a party, so I can meet her. 

In the first, we know the reason that he's having a party.  In the second, we have no idea. 

_______________ 

* This "always" isn't quite true.  Some style guides suggest that short, well-balanced clauses don't need this comma.  It's confusing but it's common.** 

** The clauses "it's confusing" and "it's common" are short and well balanced.

  • Thanks for your example with the party! It's really helpful for me:) Just to make sure that I understood the topic, could you clarify if I am right in the following example. "I had just moved to the UK, so I couldn't find anyone who wanted to travel with me." In this case "so" is a coordinating conjunction so it's right to use a comma. Using no comma is a mistake, isn't it? – Vladimir Nazarenko Jan 12 at 20:28
  • And yes, I know that I should have put comma in my pre-last sentence:) – Vladimir Nazarenko Jan 12 at 20:35
  • I'd say you understood my stance on the topic perfectly, @Vladimir. – Gary Botnovcan Jan 12 at 21:51
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A comma is only placed before conjunctions like and and or in lists, the so-called "Oxford comma".

The comma before so is a little different. With a comma, the sentence means roughly "Exercise causes blood vessels to open up -- and therefore blood flows more easily." Without a comma, it's "Exercise causes blood vessels to open up with the goal that blood flows more easily."

  • It means that it's up to us putting comma there, right? – Thanhgiang Sep 12 '17 at 3:17

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