I usually 100% of the time put a comma here. Since there is the conjunction being "but" and a subject straight after being "they" meaning that it's essential to put a comma, however, I've been told a comma is not necessary here. I think they are wrong but Grammarly also doesn't add this. Can anybody explain this?

"Boeing is known for airplanes(,) but they actually have a long history in space."

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    The use of commas is often a matter of style. There's nothing wrong with omitting the comma here but that might conflict with the requirements of your Style Manual (if one applies to you). – Chappo Says SE Dudded Monica Jun 19 '19 at 0:59
  • I understand commas can be used for style, but they also have mandatory uses in some scenarios. this is not a style manual scenario. I just don't understand why this is omittable in this circumstance. Since it goes against my teaching. If I'm wrong I want to know why so I can fix it – Toni Jun 19 '19 at 1:48
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    You can choose to omit it because there's no "rule". Some people teach that you must always put a comma before but, but others will tell you that you don't have to. There is no universal rule. Does the sentence make sense (and the meaning isn't altered) without the comma? If yes, then it's optional. Does your teacher / institution / publisher require you to follow a style guide or instruction that mandates a comma before but? If yes, it's not optional. I can't be any more help than that. – Chappo Says SE Dudded Monica Jun 19 '19 at 2:10
  • Could you add a reference for the teaching you're talking about? I've never heard of an English rule that makes a comma "mandatory" at any time. To my knowledge a comma may be desirable or undesirable, helpful or confusing, but there are no set rules. – JDM-GBG Jun 19 '19 at 2:11
  • Not sure if I'm allowed to link here, however, my past college experience and recently an Academic English: Writing Course I completed on Coursera from the University of California, Irvine. Stressed the importance of mandatory commas in certain areas. And the course would not let you continue other parts of the course if your use of mandatory comma scenarios was insufficient. You can try it for 7 days free on the platform. – Toni Jun 19 '19 at 16:58

This is a matter of style. If you always use a comma, then it's probably easier for you to continue to do so for the sake of consistency.

But, for instance, The Chicago Manual of Style, 6.22, says this:

When independent clauses are joined by and, but, or, so, yet, or any other coordinating conjunction, a comma usually precedes the conjunction. If the clauses are very short and closely connected, the comma may be omitted (as in the last two examples) unless the clauses are part of a series. These recommendations apply equally to imperative sentences, in which the subject (you) is omitted but understood (as in the fifth and last examples).

      We activated the alarm, but the intruder was already inside.
      All watches display the time, and some of them do so accurately.
      Do we want to foster creativity, or are we interested only in our intellectual property?
      The bus never came, so we took a taxi.
      Wait for me at the bottom of the hill on Buffalo Street, or walk up to Eddy Street and meet me next to the Yield sign.
      Donald cooked, Sally poured the wine, and Maddie and Cammie offered hors d’oeuvres.


      Electra played the guitar and Tambora sang.
      Raise your right hand and repeat after me.

Not only does CMOS point to some situations where the comma may be omitted (and what qualifies as "very short and closely connected" is a matter of opinion), it also qualifies its statements about the use of commas with the words usually and recommendation.

In other words, there is no absolute rule. The use of a comma in the example sentence in the question is probably more common than not—but the lack of a comma isn't actually wrong, unless you are following a specific style guide or other form of prescriptive grammar that says to never omit it. But, in that case, it's only wrong according to that particular guidance.

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