_trumps.widths.scss

This module goes hand in hand with the layout module. It serves for telling each .layout__item its width within the grid. As long as it's not .layout--auto, every .layout__item needs a width class.

<div class="layout">
    <div class="layout__item  1/3">
        Foo
    </div><!--

    --><div class="layout__item  1/3">
        Bar
    </div><!--

    --><div class="layout__item  1/3">
        Baz
    </div>
</div>

Here, we are telling each .layout__item to take up a width of 1/3 (33.333%).

To have a list of all available width classes, have a look at the source code.