Just happened to be a week where I figured out a lot of LaTeX things at once — and I wanted to get them down before they are lost forever.

I have wanted to know how to do an automatically variable height “evaluated at” symbol in LaTeX for a long time. Yesterday, I stopped waiting. First a couple of wrong ways:

\frac{x^2}{2} |_a^b \frac{x^2}{2} \right |_a^b \frac{x^2}{2} \Big |_a^b |

The right way to do it is to use the \left \right tags — but I’d only used those when I had matching symbols, i.e.:

\left ( x^2 \right ) |

Turns out, you need both tags written like the following:

\left. \frac{x^2}{2} \right |_a^b |

I found out how to do this by looking at someone else’s code, and curiously, they had a “\left.” — a “left” with a period after it — the period didn’t show up, so I didn’t know why, and my LaTeX compiled fine both including and removing the period. But the engine that typesets this page NEEDED “\left.” for it to parse properly!