Pulse-echo is any propagation mode where the transmitted beam and the reception beam follow the exact same path in the inspected piece. Pulse-echo modes can be achieved in a direct path (no skip) or with multiple skips. They are defined by the mode of the acoustic wave, longitudinal or transverse (L or T), of each leg of the propagation path: LL, TT, LLLL, TTTT, etc. Self-tandem mode transmission and reception propagation paths do not match exactly, but the transmitting and receiving elements are on the same phased array probe. In its simplest form, one segment of the propagation path, either the transmission or reception path, skips at the bottom of the inspected piece, and the other propagation path, respectively the reception or the transmission path, is direct. The detection zone occurs at the crossing of both paths. Like pulse-echo modes, self-tandem modes (or wave sets) are defined by the mode of the acoustic wave of each leg of the propagation path: TTT, TLT, etc. Self-tandem propagation modes also include wave sets with multiple skips, such as the TTTTT. Pitch-catch mode is defined in the same way as the self-tandem mode, only with the distinction that the transmitter and receiver elements are on two different phased array probes. Pulse-echo and self-tandem propagation modes are not specific to the TFM: they can be obtained in conventional phased array as well as the total focusing method. In pulse-echo TFM, thickness variations in the part only affect the position of back wall signals and ID-connected indications. Contrary to pulse-echo TFM modes, self-tandem TFM modes are highly sensitive to slight changes in part thickness because of the small overlap between the transmit and receive focal spots. Thickness variations as little as 5% can put self-tandem TFM indications 'out of focus,' so it is important to measure precisely the actual part thickness.