Mixin sets element width without clogging your markup. Set layout width, number of columns and gutter width.
I use it to make sites responsive when if don't need to change the amount of columns.
eg. 1218px, 978px and 768px grids.

Usage:

Full width

`.hero {
text-align: center;
@include grid(12);
}`

Two columns

`article {
@include grid(6);
}`
`article:nth-child(2n) { //for two column layout
@include clearfix; //html5 boilerplate clearfix
}`

Three columns

`a {
@include grid(4);
text-align: center;
img, span {
margin: 0 auto;
display: block;
}
img {
margin-top: rhythm(2);
}
span:nth-child(2) {
margin-top: rhythm(1);
}
}`

`a:nth-child(3n) {
@include clearfix;
}`