[sprobertson]

Here's what I think they mean: without Scss/Sass extension, to get your main div to be 4 columns wide you have to do something along the lines of `<div class="four columns"> ...`. With the ability to extend other classes, you could instead be more semantic and define your div as `<div id="main"> ...` and in your Scss file say `#main { @extend columns; @extend four; }`.

I'd personally stick with the presentation classes for most cases though. The templates likely do it this way so you can just throw the Foundation CSS on and it's ready to go.

EDIT: Thinking more about it, I think a huge advantage is that if you had an already-structured HTML file, you could write up a few @extend rules and apply a grid without having to add classes to everything. (You could use this technique to quickly throw up a grid for something like http://csszengarden.com/)

[nathos]

Not mixins, but @extend: http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#ex...

It's selector inheritance -- rewriting selectors instead of repeating properties & values.

[sprobertson]

Ah you're right, mixins seem to be more useful for specific nuggets of style. Updating my comment.