Is the construction "Since... then" correct? E.g.

Since it's a right triangle, then the Pythagorean theorem holds.

It sounds and feels wrong to me, and I think someone once told me it's wrong, but I haven't been able to find a definitive answer anywhere online. I see this construction used from time to time within mathematics.

  • Since you heard a mathematician say it, then it doesn't mean it's good English. The same goes for computer geeks, some of whom can't seem to build an English sentence that has an "if" but no "then". Aug 17, 2015 at 4:20
  • import @BrianHitchcock; Ha = "Those are basic programmers"; If youAskMe { they(); use(); a(); silly(); syntax(); } Aug 17, 2015 at 14:31
  • @BrianHitchcock I agree. But since I've heard it so many times, then it might actually not be wrong, just uncommon. Aug 19, 2015 at 17:23

6 Answers 6


Since is synonymous to because in this case. Your sentence is therefore equivalent to

*Because it's a right triangle, then the Pythagorean theorem holds.

There are two words indicating a causal relationship, because and then. That's one too many.

You can use either of these two:

Since it's a right triangle, the Pythagorean theorem holds.
If it's a right triangle(, then) the Pythagorean theorem holds.

Mixing them up (if...then versus since...)is a contamination.

  • Thanks! Would you please cite your source? And why the introduction of the synonymous "because" in your answer? Aug 11, 2015 at 23:32
  • 2
    My source is some 30 years of speaking the language, using since, if and then :-) I introduced because as a synonym because the word since has several meanings, and turn out to be quite a confusing word for some people, in part because of those different meanings. In "since...then", I suspect both since and then seem to be interpreted in their temporal sense (from a certain moment ... after that).
    – oerkelens
    Aug 12, 2015 at 5:15
  • Mr. Oerkelens was using the equivalency of "since" with "because" to demonstrate that your example equates to one that is obviously incorrect to any native speaker (for the reason he cited). Aug 12, 2015 at 8:21
  • @BrianHitchcock Yes, that seems likely. But I don't think the version with "because" is more obviously wrong than the once with "since". Maybe it's because I've heard it too much, but it's not obvious to me that either are wrong, and I am a native speaker. Often things that feel wrong to me only do so because I rarely hear them. The point is that I'm still not fully convinced that it (either) is (are) wrong and would therefore like an authoritative answer. Aug 12, 2015 at 20:29

I agree with @oerkelens's answer, except that I would make an allowance for since...then... in cases where there is a lot of verbiage between the two words. such as a list of reasons, a list of steps in the reasoning, or parenthetical statements.

So if St. Paul had followed his usual style, this sentence:

Therefore, since we have so great a cloud of witnesses surrounding us, let us also lay aside every encumbrance...

might have looked like this:

Therefore, since we have so great a cloud of witnesses surrounding us, including Abel, who offered to God a better sacrifice than Cain, through which he obtained the testimony that he was righteous, God testifying about his gifts, and through faith, though he is dead, he still speaks, then let us also lay aside every encumbrance...

  • 1
    +1 for threatening to overflow my English parsing stack. :)
    – tchrist
    Aug 14, 2015 at 13:08
  • @tchrist “Wouldn’t the sentence ‘I want to put a hyphen between the words Fish and And and And and Chips in my Fish-And-Chips sign’ have been clearer if quotation marks had been placed before Fish, and between Fish and and, and and and And, and And and and, and and and And, and And and and, and and and Chips, as well as after Chips?” - See more at: marginalrevolution.com/marginalrevolution/2013/04/…
    – JeffSahol
    Aug 14, 2015 at 13:40
  • Good point. However, it's still a bit of a mismatch between since implying that the condition is true and then usually treating it as hypothetical. Therefore would be much better for this purpose. All considered, I would say it's borderline grammatical when used in this way.
    – user86291
    Aug 14, 2015 at 14:17

As has been remarked "Since p, then q" is a corruption of "if p, then q". The latter is a hypothetical sentence (i.e. it expresses a hypothetical relationship between two states of affairs). "Since", however, is a premise indicator; you'd use it in an argument that is based in the actual state of affairs.

You can see both at work here: "We agreed two weeks ago that if I clean, (then) you clean. Since I in fact cleaned, you must clean."

I suppose that people might mix them up because the difference between hypotheticals and arguments is a subtle one and there are many logical parallels between the two.

I checked out the relevant definition of "then" in the American Heritage Dictionary: "As a consequence; therefore". Using substitution we can see how awkward the "since ..., then ..." construction is: "since [state of affairs], as a consequence [state of affairs]". It just strikes my native-speaker intution that "as a consequence" is preceded by a sentence expressing a state of affairs that is not in a subordinate clause.


Since is never used with then.

Since has two meanings: "for the reason that" and "happening at a time subsequent to a reference time".

For the situation you describe, which uses the first meaning, think about rearranging the sentence: The Pythagorean theorem holds, since it's a right triangle.

In the second case, it's very obvious no "then" belongs: Since 1492, the ocean has been blue.

  • But even an "If A, then B" sentence sounds wrong with "then" included when the conclusion is put before the premise; compare "B, if A" with "Then B, if A". Aug 20, 2015 at 6:46

The construction 'since A , then B' seems fine to me.

'Since' is being used as a shorthand for a modens ponens type argument.

Premise 1. The Pythagorean relationship holds for all triangles.
Premise 2. (since) this is a right triangle.
Conclusion: the Pythagorean relationship holds (for this triangle)

Premise 1 is not explicitly stated , it is implicitly assumed. "Since this is a right triangle, its sides satisfy the Pythagorean relationship "

Another example: "Since I am at least 18 years of age and a citizen of United states, I can vote." Here again we have an implicit syllogism.

  1. All people who are 18+ and citizen can vote.
  2. I am a person that is 18+ and a citizen. Conclusion : I can vote

I don't see how 'since' can mean the same thing as the hypothetical indicator 'if' used in conditional 'if-then' statements. 'Since' has a meaning that something in fact did happen. The 'if' part of a conditional does not in fact need to occur or be true. But when you say, 'since x, then y' you mean since x did in fact happen, y has to occur.

We can be even more explicit. Any 'since x then y' statement can be rewritten as "since x is the case in fact, then by logical necessity y is the case"

"Because it is a right triangle, then by logical necessity this triangle satisfies the Pythagorean relationship a^2 + b^2 = c^2"

I don't think we need to complicate issues by bringing in causality here, because showing cause and effect is a different matter. A right triangle does not 'cause' a theorem to be true in the sense of an agent cause, like my kicking a ball causes it to move. It is more like a matter of logical necessity; e.g. as a result of how we define right triangles on a Euclidean plane the Pythagorean relationship holds for all right triangles. Because x is an even integer, then x^2 is an even integer because of how we define even integers and the properties of algebra. Not because integers physically cause their squares to become even by some sort of phenomenon.

So to summarize 'since x, then y ' is equivalent in meaning to 'As a result of x being in fact true, then by logical necessity y must be true as well, (because we know that y is true whenever x is true)'. The last part in parentheses is often omitted.

  • 1
    This doesn’t actually seem to address the question at all, despite its length and detail. Nobody is doubting what since A, then B means, nor why and how it differs from if A, then B. The question is simply to do with whether or not it is grammatical to have the word there on the surface in the matrix clause. Aug 17, 2015 at 16:54
  • I may have misunderstood the question. Is the question whether the form 'since A, then B' is the same as 'since A, B'. I treat them as equivalent. You can leave out the comma if you use 'since A then B', but for readability 'since A, B' a comma should be used. Also what do you mean by 'matrix clause'.
    – john
    Aug 17, 2015 at 21:42
  • Yes, that is the question—or rather, whether it is even grammatical to have the then there at all (some answers on the page say no; I would say grammatical, but clumsy-sounding in most contexts). A matrix clause is basically the same as a main or independent clause, here (then) B, as opposed to subordinate clause, here since A. Aug 17, 2015 at 21:45

I don't think the usage is correct. Not citing any sources. Haven't done any research either. Yet, all I have to say is that instead of

Since . . . , then . . .

it should be

Since . . . , therefore . . .

In a sentence, the word since implies beginning of a cause / reason / argument. When you have stated your argument / the cause / a reason, you would follow it with your conclusion / effect / action.

What follows then in an English sentence in general is the decision to be made when a condition has been met. That is why then is a regular member of sentences beginning with if and not with since. For example:

If the answer is not accepted, then make the OP an offer she can't refuse.

As mentioned in other answers too here, we can simply write the conclusion / effect / action even without mentioning therefore. Explaining my answer with a therefore in the sentence puts the sentence using then in contrast.

  • displayName, if you cannot submit an answer with citations and proof of research, then you should not submit an answer. After you spend some time giving quality answers based on tangible sources, then you will be able to comment. At that point, you can cite as few sources and do as little research as you'd like -- as long as you limit your input to the comment section. Aug 18, 2015 at 17:58
  • @JakeRegier: Not that I cannot submit the answer with citations and proofs, it's just that my daily use of English language naturally led me to this answer. Since the 'Since.., therefore...' way of sentence framing was not only grammatically correct, but also colloquial, therefore I put it here without caring for research and citations. Aug 19, 2015 at 3:17
  • This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post - you can always comment on your own posts, and once you have sufficient reputation you will be able to comment on any post.
    – Dog Lover
    Aug 19, 2015 at 5:19
  • 2
    @DogLover: Not taking your point personally man. The thing is that 'English' is not a science like 'Chemistry' or 'Physics' that has to be supported by facts. English is a language, used for conveying thoughts. Therefore finding reasons to support a sentence structure is little unusual. General language usage and colloquialism is good enough argument for justification of a sentence structure. Aug 19, 2015 at 13:24
  • 1
    So you think "since A, therefore B" is correct? To me, it feels the same, and hence feels just as wrong, as "since A, then B" does. Aug 19, 2015 at 17:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.