# Correct typographic notation for variables with subscripts and parts of matrices?

I’m writing a scientific text and I’m trying to conform to the ISO 80000-2 standard regarding mathematical notation. After going through the standard there are still two situations I’m unsure about:

Situation 1):

What is the proper typesetting format for variables denoted with subscripts? For example, given the following expression:

$$x_{new} = x_{old} + 1$$

Should the “new” and “old” text be italic/bold, uppercase/lowercase etc.? Are there any rules for this? ISO 80000-2 does not mention variable names including subscripts as far as I can tell.

Situation 2):

If $$\mathbf{X}$$ is a matrix, denoted in uppercase, italic, bold. And I want to refer to the first column vector of the matrix, should that be: $$\mathbf{x}_1$$? I.e., lowercase bold italic to indicate that this is a vector, not a matrix. Or should it be denoted as $$\mathbf{X}_1$$ to indicate that the quantity is taken from something which is a matrix? Or are both notations wrong?

• $\boldsymbol{X}$ is denoted in uppercase, italic, bold. – Michael Hoppe May 7 at 10:11
• Yes, I know, I just didn’t know how to typeset it that way in the question. What latex command did you use just now to write in bold and italic in math mode? – Petahanks May 7 at 11:11
• $\mathit{\mathbf{X}}$ perhaps using \mathit{\mathbf{X}} but that's more of a LaTex stackexchange question already asked. – Roddy MacPhee May 7 at 15:41
• It was \boldsymbol{X}. – Michael Hoppe May 7 at 17:34

You are using

$$x_{new}$$


which is wrong anyways. Typeset

$$x_{\text{new}}$$


to get $$x_{\text{new}}$$.

• Ok, thank you. So you are saying then that subscripts should be written in non-italic, non-bold, lowercase letters? Or is the choice of uppercase/uppercase up to personal taste? Also, are you basing this answer on a standard you can refer me to or is this just your opinion? – Petahanks May 7 at 11:16
• I don't know the standards, but I know if you don't surround them with the \text{} part you get then treated as variable names by default in effect with LaTeX and MathJaX. – Roddy MacPhee May 7 at 15:33
• en.m.wikibooks.org/wiki/LaTeX – Roddy MacPhee May 7 at 17:47