您的答案在来源:https ://github.com/ericam/susy/blob/master/sass/susy/_media.scss#L23
// Nest a block of code inside a new media-query and layout context.
//
// $media-layout : a list of values [$min $layout $max $ie] including...
// : - one unitless number (columns in a layout)
// : - two optional lengths (min and max-width media-query breakpoints).
// : - one optional boolean or string to trigger fallback support for IE.
// $font-size : [optional] The base font-size of your layout, if you are using ems.
// : - defaults to $base-font-size
@mixin at-breakpoint(
$media-layout,
$font-size: $base-font-size
) {
// stuff
}
它也在文档中: http ://susy.oddbird.net/guides/reference/#ref-media-layouts
// $media-layout: <min-width> <layout> <max-width> <ie-fallback>;
// - You must supply either <min-width> or <layout>.
$media-layout: 12; // Use 12-col layout at matching min-width.
$media-layout: 30em; // At min 30em, use closest fitting layout.
$media-layout: 30em 12; // At min 30em, use 12-col layout.
$media-layout: 12 60em; // Use 12 cols up to max 60em.
$media-layout: 30em 60em; // Between min 30em & max 60em, use closest layout.
$media-layout: 30em 12 60em;// Use 12 cols between min 30em & max 60em.
$media-layout: 60em 12 30em;// Same. Larger length will always be max-width.
$media-layout : 12 lt-ie9; // Output is included under `.lt-ie9` class,
// for use with IE conditional comments
// on the <html> tag.