detects backward MF tones arriving from the incoming register. The incoming register detects forward MF tones arriving from the outgoing register. The outgoing register transmits the first signal, but from that point forward the outgoing register merely passively responds to the prompting driven by each signal transmitted by the incoming register. (In apt modern-era proleptic terms, the outgoing register's first push signal to the incoming register is similar to a peer-to-peer message, whereas the incoming register's pull signals, which request information from the outgoing register, are client–server, where the client is the incoming register and the server is the outgoing register.) Nomenclature R2 is a mnemonic for Regional System No. 2. Regional System No. 1, R1, was the CCITT designation for North American multi-frequency signaling (MF) practiced in the Bell System, although In practice that term is rarely used for MF signalling. Later in the 20th century, use of R2 signalling spread beyond Europe to all regions of the globe, including Mexico on the North American continent. The European market consisted of a large number of national telecommunications administrations, but unlike the Bell System, these were not usually integrated with an equipment manufacturer. Instead a large number of independent vendors of equipment emerged in Europe though the 20th century in a very competitive market, tendering for business from European PTTs. A more open set of technical standards developed, particularly under the auspices of CEPT and later ETSI. This allowed interconnection, interoperability and communication between a more diverse range of equipment types and telecommunication administrations. European equipment vendors such as Ericsson, Siemens and the European ITT affiliates, amongst others became major suppliers to telecommunication administrations around the world bringing technologies like Ericsson’s ARF and ITT’s Pentaconta crossbars to many markets. European derived standards were adopted more readily around the world than the proprietary standards of Bell System and Western Electric. As a result, R2 has even found sparse use in Canada, which is largely under the influence of Bell System standardization. R2 signalling was also used for register signaling within many European switching systems of that era, notably in Ericsson’s AR- crossbar systems. It was used for inter station communication and for communication between parent and remote switches. See also Compelled signalling. References Telephony signals