In mechanical engineering, the torque due to a couple is given by $\tau = P\times d$, where $\tau$ is the resulting couple, $P~$ is one of the force vectors in the couple and $d$ is the arm of the couple. A couple is made up of two forces of the same magnitude.
On the other hand, a moment is also given by $M = P\times d$. However, there is only one force involved! How can the resulting torque be of the same magnitude if in one case two, in the other case only one force is involved?