GoKawiilWikipedia entry details origins and evolution of Hayes AT command set for modems
The Hayes AT command set, originally built for 300 bit/s modems, expanded as vendors like USRobotics and Microcom added proprietary extensions using different prefix characters. Following the launch of the SupraFAXModem 14400 and subsequent market consolidation, most vendors re-standardized around Hayes's extended command set, which remains the basis for how the vast majority of dial-up modems are controlled today.