plural vs singular

So... hmmm, which one?!
waistlines?

diameter?

covers?

I don't know. Give me the answer. So that I'll learn. Thanks!

New2grammarCompanies are measuring the waistline/waistlines of their employees.

We are measuring the diameter/diameters of the *****.

We are changing the dust cover/covers of these books.

I'd use the plural for all the sentences because the employees, the ***** and the books have different waistlines, diameters and covers respectivel

Yong Liat,I love your explantion for the plural. I notice you said "I'd". Does it mean the singular is also acceptable? If so, how do you interpret the singular.

The same advice applies here as I gave for the chirping sound of the birds.
All singular or all plural, but be aware that in relaxed speech, people don't always follow this advice. In fact, there are times when you really must mix:
the pages of the book
ten copies of the document
levels of competence
CJ

CJ, all of your examples here are in the form of many parts of one object. (many to one)

Can I say, when the form is "one to many" like the diameter of these *****, it means I'm focusing on the feature (diameter instead of say, size or weight) rather than the quantities.

For example,
If we analyze the diameter of these *****, we'll find out that the volume increases cubicly

New2grammar Can I say ... the diameter of these ***** ...

Yes. Some experts on style might even find that both items in the plural in that context sounds too "fussy", even though, technically, each ball has its own diameter.
The diameter to ball relationship is one to one, however, not one to many, even though you may express it as one parameter that ap

New2grammarYong Liat,I love your explanation for the plural. I notice you said "I'd". Does it mean the singular is also acceptable? If so, how do you interpret the singular.

To me the plural version is correct. I used "I'd" because others might disagree with me.

Thank you, CalifJim and Yoong Liat.