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London-based company?
System-driven operations?

is this an applicable rule?
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The hyphen is certainly not compulsory. It's completely optional.

In fact, iIn modern times, the poor hyphen seems particularly disliked, and there are plenty of examples of words which once upon a time had a hyphen, but which now have coalesced into a single word. In the case mentioned (noun+participle), the hyphen is almost always omitted (but a space is left), with the hyphen included only occasionally, for illustrative purposes, or where its omission might lead to confusion.

Rommie.
Comments  
using hyphen just to make two words into one as a compound adj. Whether it is a must is not that I know of
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 rommie's reply was promoted to an answer.
Which of these would be correct, speaking of a type of event?


1. Climate change related event

2. Climate change-related event

3. Climate-change related event

4. Climate-change-related event
J
RobboeLondon-based company?
System-driven operations?

1. Climate change related event

2. Climate change-related event

3. Climate-change related event

4. Climate-change-related event

is this an applicable rule?

Generally hyphen is not used. Hyphen exists only when a term is used so frequently that hyphen comes naturally. What frequency has to be I do not know, but it has to be used so frequently to survive as an expression on its own. There are exceptions.

1. A climate change related event Good

2. A climate change-related event Not only bad but stupid

3. A climate-change related event Possible, but not in current use

4. A climate-change-related event Theoretically possible, but to connect more than one adjective with hyphen requires special condition, here none of them is met

London-based possible and in use, not so frequently though

System-driven possible and in use, I found examples even in titles like System-Driven but that is considered incorrect

Sometimes you have to use hyphen: mother-in-law, three-level...

Interesting example: if all items are black and white you can use black-and-white, but if each thing is only one color you cannot.

Adjective that came from phrasal verbs are regularly written with a hyphen: dried-up, broken-down... if the first part ends in 'ing' 'er' 'ed' or 'en'. The others related to phrasal verbs are written as one word, with hyphen or both.

Some adjective are written with hyphen if they come in front of a noun, but as two words in front of be

I have a brand-new Mercedes.

My Mercedes is brand new.

If one inner adjective is more related to the word before a final noun ( for example defining a type) you can use hyphen as well

a stained-glass window

a spare-time occupation

(So if we use this rule we could say for example a climate-change event)

and that would be probably all about adjectives and hyphen

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