> **Note**: Class types are used to describe values that are instances of a class---these are the most commonly used types. To instead learn about types for class objects themselves, see [T.class_of](/docs/class-of).| sorbet.org
## Why does Sorbet think this is `nil`? I just checked that it's not!| sorbet.org
A solid grasp of the tools Ruby provides for inheritance, like include and extend, helps write better code. But the concepts are often learned hastily—this post revisits them in depth.| blog.jez.io