I've been puzzled a lot on when to use a hyphen in compounds words such as cross-section, time-of-flight, state-of-the-art etc. I am writing scientific documents and I haven't found a definite rule on the use of a hyphen.

However, I came across a general rule according to which such words have to be hyphenated when they come before a noun, otherwise the hyphen is not necessary (or perhaps it's even wrong?). A few examples follow

Example 1

Cross-section measurements of nuclear reactions.


Measuring nuclear reaction cross sections.

Example 2

The time-of-flight technique


The particle has a small time-of-flight.

words have to be hyphenated when they come before a noun

Is there a rule that can be applied in such case or the use of a hyphen is interchangeable?

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2 Answers 2


As a nuclear physicist who has used and measured many cross sections, the examples are correct. Cross-section measurements (an adjective) versus a measured cross section (a noun).


The most important thing in writing is that you are understood. People need to know that a compound word is in fact is a compound word if they are to understand it correctly. For compound words which are not frequently used or could easily be mistaken for something else, you should use hyphens.

When there are 3 or more words connected together, I'd use hyphens. In the case of cross-sections vs cross sections, many people are familiar with the term so you could take out the hyphen, or you could leave it in to be more clear. Just be consistent in your decision to keep/leave it.

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