The complementiser that is the first word in a complement of a noun. When can it be omitted?
Do 1 and 2 below differ?

I am asking in general, but I exemplify with Prof John Lawler's examples modified by me:

1. He concealed the reason that he had robed.
2. He concealed the reason he had robed.

Please exclude and omit reason why, about which I ask not because 'why' is redundant.

See [5][vi], p 176, A Student's Introduction to English Grammar (2005) by Huddleston, Pullum.

  • 1
    At first blush, I can't think of any instances where that is required in sentences of similar formation. Of course there are other formations where that is ungrammatical following the noun reason: She wouldn't listen to reason. You disobeyed me- for what reason?
    – Jim
    Apr 24, 2015 at 5:20
  • This would be helpful: dictionary.cambridge.org/grammar/british-grammar/reason Apr 24, 2015 at 20:15

2 Answers 2


Rule of thumb: You can always omit that after the reason (i.e. the reason (that)). Put it another way, wherever you can say the reason that, you can omit that.

That vs. Why: You can use that (which is optional) in place of why but only in defining clauses, and make sure that you use reason not reasons (plural) if you want to use a that-clause. (See Cambridge Dictionary Online's note below.)

Question: [W]hat are the similarities or differences between the two sentences below?

  1. He didn't tell me the reason that he wore a polka-dot dress.
  2. He didn't tell me the reason he wore a polka-dot dress.

Answer: There is no difference in meaning whatsoever.


Macmillan lists these uses of reason, among others:

1 [countable] a fact, situation, or intention that explains why something happened, why someone did something, or why something is true
reason for: The police asked her the reason for her visit.
reason for doing something: Could you explain your reasons for choosing this particular course?
reason why: The reason why so many people caught the disease is still not clear.
reason (that): The reason these cars are so expensive is that they are largely built by hand.

Cambridge Dictionary Online lists reason why, reason that, and reason + to-infinitive.

Reason why We use reason why before a clause. We often omit why, especially in statements:
   The reason (why) I didn’t contact you was that I was only in town for a few hours.
Reason that We use reason that before a clause. We often omit that, especially in statements. Reason that is less common and slightly more formal than reason why:
   The reason (that) we need new guidelines is that the present ones are just not working.
 We do not use reason in the plural with a that-clause:
   There are several reasons why I don’t like the book.
   Not: There are several reasons that...

Reason + to-infinitive We can use reason with a to-infinitive:
   There’s no reason to be suspicious – everything, is perfectly normal.

Longman English Grammar (by L. G. Alexander) mentions reason why, reason for which, and reason that shortly.

1.38.3 Reason
​   That's the reason (for which) he dislikes me.
​   That's (the reason) why he dislikes me.
​   My success in business, (the reason) for which he dislikes me, has been due to hard work.
​   My success in business, the reason why he dislikes me, has been due to hard work. (The reason cannot be omitted before why.)

1.38.4 ('That') in place of 'when', 'where', 'why'
That is possible (but optional) in place of when, where and why but only in defining clauses:
​   [...]
​   That wasn't the reason (that) he lied to you ((That) can be replaced by why or for which)

Even though you've asked specifically for the exclusion of reason why, I decide to keep all alternatives--reason why, reason that, reason for, reason for doing something, reason to do something--in the above quoted texts, because it could, I believe, make this question more useful for the future reader.


Other than 'why', the sentence with 'that' seems a poor choice in this context. That is because the first clause says that he did not tell you; and you actually wanted a reason for that. It is not a statement where that works with 'the reason'. See this -

We aren't going for the simple reason that we can't afford it -- the statement type structure

As for the sentence 2, I'd not prefer that at all. I'd use the format of the reason for doing something as OALD tells.

reason (for doing something): I have no particular reason for doubting him.


He didn't tell me the reason for wearing a polka-dot dress.

  • I apologise for any offense, but I removed your writing on 'reason why' because I should like this to be excluded. I wrote this in my original OP but strengthened my diction now.
    – user8712
    Apr 24, 2015 at 13:41

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