Buying a chain - all you need to know?

[Handlebars]

I know of one replacement chain that works on my 7 speed cassette on my ready-built ebike; the KMC Z 7,8,9 speed narrow with 116 links. I learned that much by looking closely at the chain my bike came with and noticing it had the letter "Z" and the word "narrow" printed on links along with the KMC branding. The seller's phone staff told me any chain will work, but I didn't believe that.
I imagine there are alternatives if one knows details that would guide possible choices- for example, a longer chain could be shortened by removing some links.
So the question is, "what things can inform you of alternatives to the original chain, that will work on your bike?"

 

[fooferdoggie]

are you sure its a 7 8 9? usually 9 is a narrower chain. 6 7 8 is what you want for a 7 speed bike. you buy a chain for your number of cogs and the length you need.

 

[Handlebars]

are you sure its a 7 8 9? usually 9 is a narrower chain. 6 7 8 is what you want for a 7 speed bike. you buy a chain for your number of cogs and the length you need.

you're right! 6 7 8.

KMC Z7 6/7/8-Speed 116L Shimano HG Bicycle Chain​

 

[Handlebars]

are you sure its a 7 8 9? usually 9 is a narrower chain. 6 7 8 is what you want for a 7 speed bike. you buy a chain for your number of cogs and the length you need.

But it says "narrow" on it and so does the original.

 

[RunForTheHills]

I think the "narrow" designation is a holdover from the 5 and 6 speed days. Any 7 speed chain should work, although I think Campy freewheels have a bit narrower spacing than Shimano or SRAM.

 

[Handlebars]

I just found this video and it gives the most info I've found in a short search. It gives measurements and it confirms what both of you said. Thanks guys!

 

[Rickman1]

Thanks for sharing that vid. Very informative.