In the past letters were sealed shut by using melted wax. Opening a letter broke the seal, so if the seal was in one piece people knew their letter had not been read by anyone else. A ‘matrix’ is the object that was actually pressed into the wax. These seal matrices came to have personal designs cut into them. because of this, anyone receiving a letter might know from the seal design that it had been closed up by the person who had sent the letter.