Guide to LaTeX (4th Edition) (Tools and Techniques

146 Chapter 5. Mathematical Formulas
5.5.2 Selecting font size in formulas
It is possible to alter the font sizes that T E X selects for the various parts
of the formula. First we must explain what sizes are available in math
mode and what T E X’s selection rules are.
In math mode there are four font sizes that may be chosen, their actual
sizes being relative to the basic font size of the document class:
\displaystyle D Normal size for displayed formulas
\textstyle T Normal size for text formulas
\scriptstyle S Normal size for first sub-, superscript
\scriptscriptstyle SS Normal size for later sub-, superscripts
From now on we shall make use of the symbolic abbreviations D, T , S,
and SS. When math mode is switched on, the active font size becomes D
for displayed and T for text formulas. Their only difference lies in those
symbols that appear in two sizes, plus the corresponding style of those
superscripts and subscripts (Section 5.3.7). The larger symbols belong to
D, the smaller to T .
Starting from these base sizes, various math elements will be set in
other sizes. Once another size has been chosen for an element, it remains
the active size within that element.
The table below shows the selection
rules:
Active Fractions Super-,
size upper lower subscripts
D T T S
T S S S
S SS SS SS
SS SS SS SS
If the active size is D, size T is selected
for both parts of the fraction. That is,
for both numerator and denominator
T becomes the active size. If they in
turn contain further fractions, these
will be set in S. If the active size is
D or T , superscripts and subscripts
(exponents and indices) appear in S;
within them S is the active size and
any fractions or shiftings inside them
will be set in SS.
The T E X elements { \atop } and { \choose } are treated as fractions.
The active font size inside an array environment is T .
The smallest available math font size is SS. Once it has been reached,
no further reduction is possible, so that all subsequent superscripts and
subscripts appear in SS as well.
From the table one easily sees that
\[ a_0 + \frac{1}{a_1 + \frac{1}{a_2
+ \frac{1}{a_3 + \frac{1}{a_4}}}} \]
must appear as shown at the right.
a0 +
a1 +
1
1
a2+ 1
a 3 + 1
a 4
Such a math structure, called a continued fraction, is normally printed as