the ascii standard does not reserve any control characters for use as xon/xoff specifically. however, it does provide four generic "device control" characters (dc1 through dc4). the teletype model 33 asr adopted two of these, dc3 and dc1, for use as xoff and xon, respectively. this usage was copied by others, and is now a de facto standard. the keyboard equivalents of for xoff, and for xon, also derive from this usage.