AT-cut crystal unit (a crystal unit that oscillates mainly in the MHz band) oscillates in the fundamental mode and in the overtone mode which is an odd multiple of the fundamental.
Especially in the case of the third overtone, the oscillation may occur in the third overtone even though the oscillation is desired by the fundamental mode.
The following shows the difference in vibration method between the overtone orders of the AT-cut crystal unit.

If the negative resistance of the third overtone is sufficiently low relative to the negative resistance of the fundamental mode, there is almost no possibility of oscillation in the third overtone, so we conduct comparative verification of them as necessary.
If it is judged that there is a possibility of third overtone oscillation as a result of the comparison and verification, the value of the damping resistance (Rd) is increased for improvement.
(Also, if there is no pattern of Rd, we will propose to add it.)

When measuring the negative resistance of the third overtone, a fundamental crystal unit of the same frequency as the third overtone is used instead of the crystal unit mounted on the circuit. (Others are the same as the measuring method described in What is "Negative Resistance" (2) )