# Which article to use: 'a' or 'the?

In the following sentence, which article, a or the, is suitable before diameter, tensile strength, and modulus, respectively? What is the reason for that?

For example, multi-walled nanotubes with a diameter of 200 nanometers have a tensile strength of 7 GPa and a modulus of 0.6 TPa.

• I'm sure that someone will compose a well written, full explanation to this, but in short, the answer is "a" in all three cases. The other alternative would be to omit the article entirely, but including the article improves readability for me. "For example, multi-walled nanotubes with diameter 200 nm have tensile strength 7 GPa and modulus 0.6 TPa." – Jason Patterson Nov 25 '14 at 14:50
• Idiomatically, a is far more natural than the in such contexts. But repeated occurrence of the a [noun] of [adjectival quantifier] can look a bit "wordy", so it's quite common to omit both highlighted elements and just use "...nanotubes with diameter 200 nanometers have tensile strength 7 GPa and modulus 0.6 TPa". – FumbleFingers Reinstate Monica Nov 25 '14 at 15:21

'a' is suitable for all three, because all three are concepts and multiples, rather than a single specific object.

The simplest explanation is to use "the" for things which are singular. A diameter is a property of a nanotube...Since there are many nanotubes with many diameters (even if they are all the same size, each has it's own diameter), there are many diameters.

The only exception to this would be when we are talking about "The diameter" of a single nanotube... because now there is only one, and it is the subject of our sentence. Note that this only applies where we know there is only a single diameter.

The diameter of the nanotube is 200 nanometers

We may still use "a" in the following sentence:

The nanotube has a diameter of 200 nanometers

This is because in theory a nanotube may have multiple diameters, and (unlikely in the previous stencence) we have not specified that there is only one.

If we did know that it only had a single diameter, we would likely use the earlier sentence, or could construct it as:

The nanotube's diameter is 200 nanometers

The nanotube has the diameter 200 nanometers

The first of these is good, the second sounds a bit "wrong" while being technically correct.