Colloquial usage of the term 'PSK31' in amateur radio usually implies the use of the most commonly used variant of PSK31: binary phase shift keying (BPSK). The BPSK variant of PSK31 uses no error control. QPSK31, the variant based on quadrature phase shift keying (QPSK), uses four phases instead of two. It is simple to switch from BPSK to QPSK if difficulties arise during a contact; the mode has the same number of symbols per second, and hence the same bandwidth as the BPSK variant. In a coherent receiver, the bit error probability of QPSK is the same as for BPSK operating at the same power, making QPSK31 the generally preferable mode from a robustness, and thus reach, point of view.