V.26, V.26bis, V.26ter
V.26 is an older ITU standard for synchronous transmission for modems with 2,400 bit/s full duplex over leased four- wire lines.
In V.26, two bits are combined to form the dibits 00, 01, 11, 10 and transmitted with quaternary signals that differ in phase differences of 45°, 135°, 225° and 315°.
V.26bis is largely the same as V.26, but transmission is via dial-up lines.
V.26ter is from 1984 and supports asynchronous and synchronous transmission with DPSK modulation, Differential PhaseShift Keying (DPSK), via dial-up lines.