# comma with apposition depending on article? A/The maximum..., a_max,

#### cornetto

##### Member
Hello.

The maximum acceleration, a_max, is calculated by assuming a heigth h.

Rule: I need the comma when I have a definite article (The maximum acceleration, a_max,...)
No comma is need when i have an undefinite article (a heigth h).

Is this correct??

Thank you a lot.

• #### Cypherpunk

##### Senior Member
Where did you hear this rule, cornetto? Commas are used to include additional information that is not necessary for the sentence to make sense (a_max). The only thing that should have commas in this sentence is the extra information. If you leave 'a_max' out of the sentence, it still makes sense. We just don't know how to refer to 'maximum acceleration'.

#### Matching Mole

##### Senior Member
Technical writing in mathematics and physics generally follows its own rules in regard to the use of punctuation, etc. I would suggest looking at textbooks on the subjects and see how information like this is presented. I think unnecessary commas should be avoided, and I believe, although I am not even remotely an expert on the subject, that your sentence would not be punctuated at all:
The maximum acceleration a_max is calculated by assuming a height h.

Here is a similar authentic example from the web:
It should be noted that if the maximum acceleration A_max (T) and the maximum velocity V_max (T) were not dependent on the temperature, [...]
I agree that the rules of commas do not depend on articles. There are plenty of resources on the usage of commas, such as this one, which I would recommend taking a look at.

(Note the correct spelling of "height".)

#### cornetto

##### Member
Thank you. I really appreciate it.

Variation of r_d with depth z, in metres, and earthquake
magnitude M_W. (In a caption)

What do you think about that?

I have read a lot of "comma-rules websites" but it is not always easy to determine if a information is necessary or not.

#### Matching Mole

##### Senior Member
I think it's fine, but captions tend to omit unnecessary grammatical detail, as do headlines, titles etc. Captions are also often not complete sentences (as in this case), but the same rules do not apply to them as they do in normal writing. I think you could put:
Variation of r_d with depth z metres and earthquake magnitude M_W