## how I can use a vertical bar to represent evaluation in LaTeX

Hey guys. This might not be the right place for this, but any ideas on how I can use a vertical bar to represent evaluation in LaTeX?

Example

$$\displaystyle \frac{d}{dt} |_{t=0} f(t)$$

I would like the vertical bar to be the size of the differential. I've tried using \left|, \right|, \vert, etc. but nothing seems to work. Thanks a lot in advance.

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 Recognitions: Homework Help Hi Kreizhn, Is this it? $$\left. \frac{d}{dt} \right|_{t=0} f(t)$$ which is given by \left. \frac{d}{dt} \right|_{t=0} f(t) Using either \left or \right on a period means the automatic delimiter sizing takes place, but only one delimiter is shown.
 Excellent, thank you

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## how I can use a vertical bar to represent evaluation in LaTeX

\frac{d}{dt} \bigg|_{t=0} f(t) achieves a similar effect
with manual sizing (by using two g's) and the unneeded \left. tag.

$$\frac{d}{dt} \bigg|_{t=0} f(t)$$

 I'm using lyx and I'm having difficulty trying to find the code for this vertical line. Any suggestions? I use the | but it's extremely small.
 Maybe try \|? The height of the vertical line will be determined by the {stuff} in between the \left. {stuff} \right|. If it's not naturally big, you may want to add an "invisible tower." My invisible tower is called \xstrut, and is defined as follows \newlength{\myVSpace}% the height of the box \setlength{\myVSpace}{3ex}% the default, \newcommand\xstrut{\raisebox{-.5\myVSpace}% symmetric behaviour, {\rule{0pt}{\myVSpace}}% } To change the height of the box, change \setlength{\myVSpace}{Your number here}
 \mathbf{M \bigg|_{x^k} \Delta x^k = -f(x^k)} $$\mathbf{M \bigg|_{x^k} \Delta x^k = -f(x^k)}$$