Abstract: The present invention relates to an indexing head designed to print on postal envelopes indexing marks translating information written on the envelopes so as to enable the same to be read by an automatic sorting machine.The indexing code comprises printing 20 marks with actinic ink in a line comprising 30 positions. The head comprises as many reciprocal elements as there are positions, each reciprocal element being displaced by an electromagnet associated therewith. The printing operation is effected by selectively exciting the electromagnets corresponding to positions where the marks are to be printed.The indexing head is designed for use in indexing apparatus preceding automatic sorting of mail.