Wikipedia:Categorization dos and don'ts

Categories are a vital tool for navigating Wikipedia.
When categorizing pages:
Use the most specific categories possible.
Categorize based on defining characteristics.
Add pages to multiple categories in overlapping trees.
Provide sort keys or a {{DEFAULTSORT:}} key.
Add cleanup templates where needed.
Don't add pages to non-existent categories.
Don't categorize based on trivial characteristics.
Don't add pages to non-neutral or unverifiable categories.
Don't categorize fictional subjects together with real ones.
Don't apply maintenance categories directly: use templates.

See also