The slash has become standard in several abbreviations. Generally, it is used to mark two-letter initialisms such as A/C (short for "air conditioner"), w/o ("without"), b/w ("black and white" or, less often, "between"), w/e ("whatever" or, less often, "weekend" or "week ending"), i/o ("input/output"), r/w ("read/write"), and n/a ("not applicable"). Other initialisms employing the slash include w/ ("with") and w/r/t ("with regard to"). Such slashed abbreviations are somewhat more common in British English and were more common around the Second World War (as with "S/E" to mean "single-engined"). The abbreviation 24/7 (denoting 24 hours a day, 7 days a week) describes a business that is always open or unceasing activity.