A guide for using @extend and/or @mixin

[BensonLiao]

Beware of the output when using @extend when you want to DRY,
you might be over engineering :(

Only use @extend when the rulesets that you are trying to DRY out are inherently and thematically related.
Do not force relationships that do not exist: to do so will create unusual groupings in your project, as well as negatively impact the source order of your code.
Use a @mixin to either inject dynamic values into repeated constructs or as a Sassy copy/paste which allows you to repeat the same group of declarations throughout your project while keeping a Single Source of Truth.

-- csswizardry

when-to-use-extend-when-to-use-a-mixin