Could someone please explain what rule governs the comma and dash usage. Why is the dash placed after the should. Couldn't the dash be placed after the 'have'?

He might – and according to plans, should – have reinforced the Second Division.

  • I would venture this is an error. There should be a comma after "and". I am unaware of any dashes rule for commas. Follow normal rules.
    – Unrelated
    Commented Jan 2, 2017 at 22:01
  • 1
    You have to be able to read the sentence two ways to have it make sense. If you put the comma after have you cannot read it like this: He might have reinforced the Second Division. You would get: /He might reinforced the Second Division/ and that is not grammatical. The test is: remove the dashes and see if your sentence should read correctly. If it does not, your dashes are in the wrong place.
    – Lambie
    Commented Jan 2, 2017 at 22:25
  • @Unrelated Why is a comma after and necessary?
    – deadrat
    Commented Jan 3, 2017 at 5:28
  • @Lambie Where did you get this rule? Zeugma is sufficient to make might and should have reinforced just fine.
    – deadrat
    Commented Jan 3, 2017 at 5:32
  • 1
    @deadrat only because of the comma after plans. The aside is and should; the comma after plans breaks this clause up unless "according to plans" is fully set apart by commas. "And according to plans should" or "and, according to plans, should"
    – Unrelated
    Commented Jan 3, 2017 at 5:58

3 Answers 3


I will offer an intuitive explanation. Here's the basic, underlying sentence:

He might have reinforced the Second Division.

Fine, so far. Let's introduce "and should":

He might –-and should-– have reinforced the Second Division.

The dashes surround the interruption "and should". If the new stuff were "and should have", then it would look like this:

He might have –-and should have-- reinforced the Second Division.

But it's rather unpleasant to have that repeated "have," so usually that's edited out. Now let's introduce "according to plans":

He might –-and, according to plans, should-– have reinforced the Second Division.

Insertions of this type are often surrounded by commas, dashes, or parentheses. But if you think the meaning will be clear you don't absolutely have to use any punctuation around them. So the following would work:

He might –-and according to plans should-– have reinforced the Second Division.


Have a look at the Wiki How page on dashes and you will see how the use of dashes work.

Wiki How says

Dashes can connect an independent clause with the 'interrupting' thought like so:

  • Independent clause—thought—independent clause.

  • Independent clause—thought.

To check the correct usage of dashes, you can take the part of the sentence out which is separated by dashes, and the sentence should make sense, just as it does with the original text. This leaves

He might have reinforced the Second Division.

If you place the dash after have as you suggest, when taking the dashed area out you would be left with

He might reinforced the Second Division.

which is wrong and indicates incorrect positioning of the dashes.

Now for the comma usage, the rules apply within dashes just as they do outside them. However, there is a slight difference when reading the whole passage.

Because of the dashes, the Grammar Book gives you a rule which applies to your passage, but in a slightly different way.

Rule 2. Use a comma to separate two adjectives when the order of the adjectives is interchangeable.

Example: He is a strong, healthy man. We could also say healthy, strong man.

Example: We stayed at an expensive summer resort. We would not say summer expensive resort, so no comma.

Another way to determine if a comma is needed is to mentally put and between the two adjectives. If the result still makes sense, add the comma. In the examples above, a strong and healthy man makes sense, but an expensive and summer resort does not.

The dashes changed the interchangeability from adjectives to verbs. In this case, the comma indicates interchangeability between the verbs might and should.

He should have reinforced the Second Division.

The rest of the dashed area tells you why he should have (because the plans should have resulted in success if they were followed correctly)

Another example would be

The mathematics student could — if he used the method he was taught, would — have got the correct answer.

The interchangeability is between could and would.

  • The mathematics student would have got the correct answer


  • Because the method he was taught would have given it.

Concrete is good when figuring out how to build a sentence:

1) He might – and, according to plans, should – have reinforced the Second Division.

2) Remove the add-on (phrasing between dashes): He might have reinforced the Second Division.

Question: Is that a load-bearing (grammatical) sentence? Answer: Yes. The dashes set off a parenthetical phrase (which is not a clause).

Check it: 3) He might and according to plans should have, reinforced the Second Division.

Grammar error: the modal: /might have reinforced/ and or /should have reinforced/. Removing the dashes and using only one comma produces an ungrammatical reading of the main clause and destroys the parenthetical nature of the phrase between commas.

Should or might in the past are: Should + have + past participle.

A parenthetical dash is used to emphasize some aspect of a sentence.

Another example: John knew - or surmised he had known - the real reason for the crime.

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