In scientific writing, small typographic choices in equations carry meaning. Italic vs. upright (roman) font is not aesthetic — it distinguishes variables from descriptive labels.
1. Variables → Italic
Any symbol representing a mathematical variable should be italic:
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Spatial coordinates: x, y, z
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Tensor components: \sigma_{ij}, \varepsilon_{ij}
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2D stress state: \sigma_{xx}, \sigma_{xy}, \sigma_{yy}
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Fourth-order stiffness: C_{ijkl}
Indices i, j, k, l are running indices → italic.
A viscoelastic example:
\sigma_{ij}(t) = \sum_{n=1}^{N} C_{ijkl}^{(n)} \, \varepsilon_{kl}(t)
Here i,j,k,l,n,N are all variables → italic.
If it varies mathematically, it is italic.
(See SI/NIST typography conventions [1].)
2. Descriptive Subscripts → Roman (Upright)
If the subscript is a material label, abbreviation, or word fragment, it should be upright:
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E_{\mathrm{f}} (fiber, matrix)
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C_{\mathrm{eff}} (effective stiffness)
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\sigma_{\mathrm{ult}} (ultimate strength)
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\varepsilon_{\mathrm{th}} (thermal strain)
Writing C_{eff} causes LaTeX to interpret it as e \cdot f \cdot f. Multiletter abbreviations should therefore be roman to avoid ambiguity [2].
3. Units → Always Roman
Units are never italic:
70\,\mathrm{GPa}, \quad 250\,\mathrm{MPa}, \quad 5\,\mathrm{mm}
Always include a space between the number and the unit (SI requirement [1]).
Quick Decision Rule
Mathematical variable or index? → Italic
Label, abbreviation, acronym, or unit? → Roman
Consistency improves readability and prevents unintended meaning.
For more detailed explanation and additional examples, interested readers may consult [3,4].
References
[1] National Institute of Standards and Technology (NIST), Guide for the Use of the International System of Units (SI) and related typography guidance.
[2] Physics/engineering typography conventions summarized in the Mathematics Stack Exchange discussion.
[3] Andrew P. Davison, Typesetting subscripts, units and physical quantities.
https://andrewdavison.info/notes/typesetting-comp-neuro/
[4] “Math typesetting conventions: two-letter subscripts in an equation,” Mathematics Stack Exchange.
https://math.stackexchange.com/questions/4635207/math-typesetting-conventions-two-letter-subscripts-in-an-equation