Have you started working with Drupal 8 yet? If so, you might have noticed that Drush 7 doesn't play nice with Drupal 8. And if you install Drush 8, that won't work with your Drupal 7 sites. Yikes!

Have no fear!

Here's how to install BOTH Drush 7 and Drush 8 AND have each project automatically use the version that corresponds to that install. It's stinkin' awesome!