It was believed that this mark was made in a hidden place of the person's body, i.e. the armpit, under the eye lip, in the genital zone, inside the mouth. It often looked like a scar or a spot with a special shape (all this subject to interpretation by the judges that examined the accused).
The other form of the diabolical mark (always present in every accused according to the tribunals that used this test) was a small zone of the body in which the individual could not feel a prick; so it was established the proof of the needle, in which the person's body was stung with a needle until that area was found. It has been proved that every person has an insensible area in his/her body, so all people put to this test were found guilty of practising witchcraft.
It shouldn't be confused with a diabolical signature.