in lib/dd.breakpoints.js [322:328]
_bpBetween = function(min, max, property) {
var bpMin = _bpToEms(min),
bpMax = _bpToEms(max, true),
bpType = (typeof (property) === 'string') ? property : 'width';
return '(min-' + bpType + ': ' + bpMin + 'em) and (max-' + bpType + ': ' + bpMax + 'em)';
};