Bootstrap, pull-left for small devices

Just add this to your SASS file:

@media (max-width: $screen-xs-max) {
    .pull-xs-left {
        float: left;
    }
    .pull-xs-right {
        float: right;
    }
}

@media (min-width: $screen-sm-min) and (max-width: $screen-sm-max) {
    .pull-sm-left {
        float: left;
    }
    .pull-sm-right {
        float: right;
    }
}

@media (min-width: $screen-md-min) and (max-width: $screen-md-max) {
    .pull-md-left {
        float: left;
    }
    .pull-md-right {
        float: right;
    }
}

@media (min-width: $screen-lg-min) {
    .pull-lg-left {
        float: left;
    }
    .pull-lg-right {
        float: right;
    }
}

Insert actual px values for $screen-* if you use plain CSS of course.

HTML:

<div class="pull-md-left pull-lg-left">
    this div is only floated on screen-md and screen-lg
</div>