text: Hahn's 1950 paper showed that another method for generating spin echoes is to apply three successive 90° pulses. After the first 90° pulse, the magnetization vector spreads out as described above, forming what can be thought of as a "pancake" in the x-y plane. The spreading continues for a time , and then a second 90° pulse is applied such that the "pancake" is now in the x-z plane. After a further time  a third pulse is applied and a stimulated echo is observed after waiting for a time  after the last pulse.