Q. Can you clarify when commas should be used with an “or” phrase? For example, should it be “Table salt, also known as sodium chloride or NaCl” or “Table salt, also known as sodium chloride, or NaCl”?

A. When “or” is used to introduce an equivalent rather than an alternative, a comma is strictly correct. Compare the following examples:

To get to the stadium, you can take a train or a bus.

To get to the stadium, you can take the elevated train, or el.

In the first example, “a bus” is an alternative to “a train”; it is not another way of writing “a train.” A comma is therefore unnecessary. In the second example, “el” is equivalent to “elevated train,” so the comma is correct. Still, such a comma may be omitted in certain cases, provided the meaning remains clear. You may want to do this, for example, to avoid comma clutter in text that mentions more than a few such equivalents. In an example like yours, which presents not two equivalent terms but three—all in a single sentence—the comma before “or” remains strictly correct. But it’s a little fussy and could be omitted if done so consistently.