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Instead of the arrow notation \vec{a} vectors in scientific journals are often represent in bold font \bold{a}.

My question is if I have a vector with a subscript is it conventional to use \bold{a}_{sub} or \bold{a_{sub}}?

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    What does sub represent in your paper? Is it an index (e.g., i) that references the i-th element of the vector a, or is sub an integral part of the overall name of the vector (e.g., a_{new} and a_{old})? – Mico May 1 '16 at 11:08
  • @Mico in my case it is the latter, i.e. an integral part of the vector name. i wouldn't mind if someone could specify for both cases though if they are different (which i assume you are implying?) – Wolpertinger May 1 '16 at 11:35
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    The reason I asked what sub represents is this: If sub is an index of some kind, it's generally not typeset in bold (assuming, of course, that a itself is rendered in bold). On the other hand, if sub is indeed an integral part of the vector's name, it may be advisable to render in in bold as well. Whatever you do in terms of notation, do ensure you're absolutely consistent. – Mico May 1 '16 at 11:35
  • Could you give an example or two of vectors-with-subscripts names? – Mico May 1 '16 at 11:42
  • @Mico that essentially answers my question i think. My examples might be not very meaningful but i can try. i've got spatial vectors associated with a wave that are evaluated in two different planes (called a for aperture and s for screen). so i end up with this like r_a, r_s where r is a 3D space vector. – Wolpertinger May 1 '16 at 11:59
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An important question to ask is, what do the subscripts represent? If "sub" is of the form i or j and serves to index elements of the vector named a, it's customary not to typeset the indices in bold. Thus, you'd write \mathbf{a}_i and \mathbf{b}_j.

If, on the other hand, "sub" forms an integral part of the name of the vector itself, it's more common to typeset the subscript part of the name in bold as well. E.g., if r is the main part of the vector and a and s serve to distinguish sub-types of r (say, for 'aperture' and 'screen'), you may want to indicate this property to your readers by typing \mathbf{r_a} and \mathbf{r_s}, respectively.

enter image description here

Two final remarks: (a) As the preceding screenshot shows, \mathbf produces upright-bold characters. If you would rather generate italic-bold characters, load the bm package and write, say, \bm{r_a}. (b) Whatever you do in terms of vector notation, be sure to choose notational consistency.

\documentclass{article}
\begin{document}
$\mathbf{a}_i$, $\mathbf{b}_j$ \quad vs.\quad $\mathbf{r_a}$, $\mathbf{r_s}$
\end{document}
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