On the use of italic and roman fonts for symbols in scientific text
Scientific manuscripts frequently fail to follow the accepted conventions concerning the use of italic and roman
fonts for symbols. An italic font is generally used for emphasis in running text, but it has a quite specific
meaning when used for symbols in scientific text and equations. The following summary is intended to help in
the correct use of italic in preparing manuscript material.
1.
The general rules concerning the use of an italic (sloping) font or a roman (upright) font are presented in
the IUPAC Green Book [1] on p.5 and 6, and also p.83 to 86 in relation to mathematical symbols and
operators (see also p.75, 76, and 93). These rules are also presented in the International Standards ISO 31
and ISO 1000 [2], and in the SI Brochure [3].
2.
The overall rule is that symbols representing physical quantities (or variables) are italic, but symbols
representing units, or labels, are roman. Sometimes there may seem to be doubt as to whether a symbol
represents a quantity or has some other meaning (such as a label): a good general rule is that quantities, or
variables, can be given a value, but labels cannot. Vectors, tensors and matrices are usually denoted using
a bold-face (heavy) font, but they should still be italic since they are still quantities.
Example: The mass of my pen m = 24 g = 0.024 kg. The electric field strength
E
has components E
x
,
E
y
, and E
z
. The Planck constant h = 6.626 068 76
(52)
×
10
–34
J s.
3.
The above rule applies equally to letter symbols from both the Greek and the Latin alphabet, although
authors often appear to resist putting Greek letters into italic.
Example: when the symbol
is used to denote a physical quantity (such as mass or reduced mass) it
should be italic, but when it is used in a unit such as the microgram,
g , or when it is used as the
symbol for the muon,
(see 5 below), it should be roman.
4.
Numbers, and labels, are generally roman (upright), since they are not physical quantities.
Example: The ground and first excited electronic states of the CH
2
molecule are denoted
…(2a
1
)
2
(1b
2
)
2
(3a
1
)
1
(1b
1
)
1
,
X
3
B
1
, and
…(2a
1
)
2
(1b
2
)
2
(3a
1
)
2
,
~
a
1
A
1
, respectively.
The
π
-electron configuration and symmetry of the benzene molecule in its ground state are denoted:
(a
2u
)
2
(e
1g
)
4
,
X
1
A
1g
. Note that all these symbols are labels and are roman.
5. Symbols for elements in the periodic table should be roman, since they are not physical quantities.
Similarly the symbols used to represent elementary particles are always roman. (See, however, paragraph 9
below for the use of italic font in chemical-compound names.)
Examples: H, He, Li, Be, B, C, N, O, F, Ne, … for atoms; e for the electron, p for the proton, n
for the neutron,
for the muon, α for the alpha particle, etc.
6. Symbols for physical quantities are single letters of the Latin or Greek alphabet. Exceptionally two letters
are used for certain dimensionless quantities, such as the Reynolds number, Re. However the symbols are
frequently supplemented with subscripts or information in brackets to further specify the quantity. Further
symbols used in this way are either italic or roman depending on whether they represent physical quantities
or labels.
Examples: H denotes enthalpy, but H
m
denotes molar enthalpy (m is a mnemonic label for molar, and
is therefore roman). C
p
and C
V
denote the heat capacity at constant pressure p and volume V,
respectively; but C
p,m
and C
V
,
m
denote the molar heat capacity at constant p and V, respectively (note the
roman m but italic p and V). The chemical potential of argon might be denoted
Ar
or
()Ar , but the
chemical potential of the ith component in a mixture would be denoted
i
, where the i is italic
because it is a variable index.
7. Symbols for mathematical operators are always roman. This applies to the symbol
∆
for a difference,
δ
for
a small difference, d for an infinitesimal difference (in calculus), and to capital
Σ
and
Π
for summation
and product signs. The symbols
π
, e (base of natural logarithms), i (square root of minus one), etc. are
always roman, as are the symbols for named functions such as log (lg, ln or lb), exp, sin, cos, tan, erf, div,