title: Why is there a /usr/local/opt directory created by Homebrew and should I use it?
body: <p>I am quite new at using homebrew and I am trying to figure out how it works to use some libraries (boost, gsl, openblas for example) in my own project.</p>
  
  <p>I have understood that each formula is installed by Homebrew in /usr/local/Cellar/ and then symlinked in usr/local/bin, usr/local/lib, usr/local/include, so it seems, excepts for keg-only formulas so it does not mess with already installed libraries by the OS (cf. <a href="https://stackoverflow.com/questions/17015285/understand-homebrew-and-keg-only-dependencies">Understand homebrew and keg-only dependencies</a> for example). But I found out that every formula is also linked to a /usr/local/opt directory.</p>
  
  <p>So my question is why is there this /usr/local/opt directory (it is kind of redundant), and what path do I have to use for using formulas (usr/local/Cellar or usr/local/ or usr/local/opt basically) ?</p>