Fractions come in two varieties: Display style and text style. A text style fraction is small enough to fit within the height of a line of text, like $\frac{a}{b}$. In contrast, a display style fraction is taller and will extend beyond the normal line height if used in text: $\dfrac{a}{b}$. A text style fraction is used automatically by LaTeX in the following circumstances:

  • In inline equations delimited by $ ... $ or $( … )$, such as $\frac{a}{b}$ ($\frac{a}{b}$).
  • In the numerator or denominator of another fraction.
  • In matrix environments, such as pmatrix or bmatrix.
  • In cases environments.

Here is a example that demonstrates text style fractions:

Code: 
\[
    \frac{1 + \frac{1}{2}}{2}
    +
    \begin{bmatrix} 
        1 & \frac{1}{2} & \frac{1}{3} 
    \end{bmatrix} 
    +
    \begin{cases} 
        2 + \frac{1}{4} \\
        \frac{1}{5}
    \end{cases}
\]
Output:
$$ \frac{1 + \frac{1}{2}}{2} + \begin{bmatrix} 1 & \frac{1}{2} & \frac{1}{3} \end{bmatrix} + \begin{cases} 2 + \frac{1}{4} \\ \frac{1}{5} \end{cases}. $$

In contrast, a display style fraction is used in the following cases:

  • In a display equation, delimited by \[...\], $$...$$, \begin{equation} \end{equation} or the other similar environments (equation*, align, and align*).
  • Anywhere (including where a text style fraction would normally be used) if \dfrac is used instead of \frac, or if \displaystyle is placed anywhere before the fraction.

When writing a fraction in LaTeX, consider the following guidelines:

  1. Do not use display style fractions inline, in text. They will look bad and cause the height of the lines to grow.
  2. Only use text style fractions for simple fractions, such as $\frac{1}{2}$. Do you want to read a fraction like $\frac{1+\sqrt{x^2}}{e^{-\frac{x}{\pi}}}$? I wouldn’t! Either make the equation a display equation (e.g., if it is in a bmatrix environment where there is sufficient space), or rewrite the equation with a slash, like $(1+\sqrt{x^2})/(e^{-x/\pi})$.
  3. If writing a fraction using the slash notation and the expression for the numerator or denominator is taller than a typical line of text, use 
     \left. <numerator> \middle/ <denominator>\right

    to ensure that the slash matches the height of the numerator and denominator. The \shortfrac macro provxided here automatically scales the slash for you.

  4. When a minus sign occurs before a fraction, it more easily lost in calculations, so consider rewriting the expression. For instance, rewrite “$-\frac{a}{b}$” into “\frac{-a}{b}”. If possible, eliminate the minus sign by distributing it to the numerator or denominator, such as “$-\frac{a - b}{c}$” changing to “$\frac{b - a}{c}$.”
  5. When practical, avoid placing fractions inside fractions. For instance, change \(\frac{1 + 1/n}{1} \quad \text{into} \quad \frac{n + 1}{n}.\)