foo.haml

#content
  .row
    .third
    .third
    .third.last
  .row
    .half
    .half.last
  .row
    .fourth
    .fourth
    .fourth
    .fourth.last

foo.sass

$blueprint-grid-columns: 24

@import blueprint

.row
  +container

.last
  +last

.section
  +column-base
  &:last-child
    @extend .last

.third
  @extend .section
  +span($blueprint-grid-columns / 3)

.fourth
  @extend .section
  +span($blueprint-grid-columns / 4)

.half
  @extend .section
  +span($blueprint-grid-columns / 2)

body.article
  #sidebar
    @extend .fourth
  #content
    @extend .half
  #right-side-callout
    @extend .fourth