\AmSTeX\ offers tfrac, frac, and dfrac, all useful.

$$
fg=\tfrac12[(f+g)^2-f^2-g^2]
$$

$$
\frac{x}{1+\frac{x}2}
\quad
\text{versus}
\quad
\frac{x}{1+\dfrac{x}2}
$$

\AmSTeX\ also offers cfrac for continued fractions.

$$
a_0+1\cfrac 1\\
a_1+\cfrac 1\\
a_2+\cfrac 1\\
a_3+\cfrac 1\\
a_4\endcfrac
$$
\bye