> ## Documentation Index > Fetch the complete documentation index at: https://docs.dairy.foundation/llms.txt > Use this file to discover all available pages before exploring further. # Generics in units At the start of looking at units, we looked at `Unit>` and `ReifiedUnit, RU: ReifiedUnit>`. Lets take a second look at why these are needed, and why they work. Firstly, these generics allow for generic family lock-in, basically, code can be written that is generic across all of `RU: ReifiedUnit`, and once you use a specific unit there, nothing else can be used, by using `RU` rather than just `ReifiedUnit`, we ensure that **only** `Distance`s can be used, as opposed to any `ReifiedUnit`, which means we can rely on the properties and functions of `Distance`. The same applies for `Unit>`. We use this cycle to define a family, like `DistanceUnit` which extends `Unit`, this means that `DistanceUnit` needs to be able to handle conversion between itself and any other `DistanceUnit` but not any `Unit`. This is a fairly complex usage of generics and type systems, so don't worry if you don't get it. The basic idea, is that we can write code that works for any '`RU`' or '`U`' and guarantee that it locks to that one, once its used.