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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.
 Download version
of this Document 
<PDF file 22 KB>; with section 9 revised
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 and matrices are usually denoted using a
boldface (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) x
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 m 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,
mg, or
when it is used as the symbol for the muon, m
(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
. . .
and
. . .
respectively.
The pelectron configuration and symmetry
of the benzene molecule in its ground state are denoted:
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 chemicalcompound 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, m for the muon, a 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 m_{Ar}
or m(Ar), but the chemical potential
of the ith component in a mixture would be denoted m_{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 D for
a difference, d for a small difference, d
for an infinitesimal difference (in calculus), and to capital S
and P for summation and product signs. The symbols p,
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, grad, curl or rot
(the operator curl or rot, and the corresponding symbol see
pdf version of this symbol may be printed boldface since it represents a vector). Some
of these symbols are also sometimes used to represent physical quantities:
then of course they should be italic, to distinguish them from the corresponding
mathematical operator.
Examples: DH = H(final)  H(initial); (dp/dt)
used for the rate of change of pressure; dt
used to denote a small time interval. But for a damped
linear oscillator the amplitude F as a function of time t
might be expressed by the equation: F = F_{0}
exp(dt) sin(wt)
where d
is the decay coefficient (SI unit: Np/s) and w
is the angular frequency (SI unit: rad/s). Note the use of roman d
for the operator in a small time interval
dt,
but italic d for the decay coefficient
in the product dt. Note that
the products dt and wt
are both dimensionless, but are described as having the unit neper
(Np = 1) and radian (rad = 1), respectively.
8. Symbols for the fundamental physical constants are always regarded
as quantities (even though they are not quite variables!) and they should
accordingly always be italic. Sometimes the fundamental physical constants
are used as though they were units, but they are still given italic
symbols. However the electronvolt, eV, and the unified atomic mass unit,
u, have been recognized as units by the Consultative Committee on Units
of the BIPM and they are accordingly given roman symbols.
Examples: c_{0} for
the speed of light in vacuum, m_{e}
for the electron rest mass, h for
the Planck constant, N_{A} or
L for the Avogadro constant, e for the elementary charge,
a_{0} for the Bohr radius, etc.
The electronvolt eV = e x
V = 1.602 176 462 (63) x
10^{19} J, the symbol eV is roman.
9. (revised by ICTNS in May 2007)
Greek letters are used in some cases for certain purposes in systematic
organic, inorganic, polymer, and biochemical nomenclature. These should
be in roman (upright) type. They designate e. g. the position of substituents,
double bonds, ligatingatom attachment and bridging mode in coordination
compounds, end groups in structurebased names for polymers, and configuration
in carbohydrates and natural products.
Letter symbols for elements are italic when used in names indicating
attachments to heteroatoms, e.g., O, N, S, and
P. The italic element symbol H denotes indicated or added hydrogen.
See references [4] and [5].
Examples:
(whydroxyalkyl)benzene 
1,6dimethylD^{1}heptalene 
tetracarbonyl(h^{4}2methylidenepropane1,3diyl)chromium

dimchloridobis(dichloridoaluminium) 
aethylcyclopentaneacetic
acid

bmethyl4propylcyclohexaneethanol

[N, N'bis(2aminokNethyl)ethane1,2diaminekN]chloroplatinum(II)

tetracarbonyl(h^{4}2methylidenepropane1,3diyl)chromium

a(trichloromethyl)wchloropoly(1,4phenylenemethylene)

aDglucopyranose

5aandrostan3bol

Nmethylbenzamide 
Oethyl hexanethioate 
3Hpyrrole 
naphthalen2(1H
)one 
bis(tetrabromidorhenate)(ReRe)(2)

I. M. Mills and W. V. Metanomski, Interdivisional Committee on Nomenclature
and Symbols, IUPAC, December 1999.
References:
[1] Quantities, Units and Symbols in Physical
Chemistry, the IUPAC Green Book, edited by I. Mills, T. Cvitas,
K. Homann, N. Kallay, and K. Kuchitsu, 2nd Edn., Blackwell Science,
Oxford 1993.
[2] The ISO Standards Handbook, Quantities
and Units, ISO, Geneva, 1993.
[3] Le Système International d'Unités
(the SI Brochure), 7th Edn. (French and English), BIPM, Sèvres,
1998.
[4] Principles of Chemical Nomenclature,
a guide to IUPAC recommendations, G.J. Leigh, H.A. Favre, and W.V. Metanomski,
Blackwell Science, Oxford 1998.
[5] A Guide to IUPAC Nomenclature of Organic
Compounds, R. Panico, W.H. Powell, and J.C. Richer, Blackwell Science,
Oxford 1993.
Page last modified 14 May 2007
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