# How to vertically align underbraces

I am trying to balance underbraces in the equation below, how can I ensure that both underbraces at the same depth?

$$y = \underbrace{ \left [ x^2 + r^2 (a^2+\frac{a^{0.3}}{b}) \right]}_\text{growth rate} \times \underbrace{ C_0}_\text{initial}$$
• Jan 16, 2014 at 17:06

One way to solve this is to insert a "vertical phantom" -- an object with a certain height and depth but no width and therefore invisible, hence its name "phantom" -- in the second \underbrace expression.

In the case at hand, an immediately ready argument for such a \vphantom is the tallest "math molecule" from the first \underbrace expression; this turns out to be the term \left(\frac{a^{0.3}}{b}\right).

I would also like to suggest that you encase both \underbrace expressions in their entirety in curly braces; this'll improve the horizontal spacing around the \times symbol.

Addendum, Dec. 2019: The word "initial" is wider than the material immediately above it. This causes an unnecessarily wide gap to open up between the material in large square brackets and C_0. In the second row of the following screenshot, a \mathclap directive is used to set the width of "initial" to 0, leading to a better amount of horizontal spacing.

\documentclass{article}
\usepackage{mathtools} % for '\mathclap' macro
\begin{document}
\begin{align}
y
&= {\underbrace{%
\left[ x^2 + r^2 \left(a^2+\frac{a^{0.3}}{b}\right) \right]
}_{\text{growth rate}}}
\times
{\underbrace{%
\vphantom{ \left(\frac{a^{0.3}}{b}\right) }
C_0}_{\text{initial}}}\\
&= {\underbrace{%
\left[ x^2 + r^2 \left(a^2+\frac{a^{0.3}}{b}\right) \right]
}_{\text{growth rate}}}
\times
{\underbrace{%
\vphantom{\left(\frac{a^{0.3}}{b}\right)}
C_0}_{\mathclap{\text{initial}}}}