Means to control feeding of a multi-strike ink ribbon (19) housed in a reversible cassette (12) is disclosed. The cassette carries a magnetisable element (20; 30,31) and when the cassette is inserted in printing apparatus (10, 26) the element is located in operative alignment with a magnetic sensing device (21; 32,33) and with an electromagnet (25; 34). Upon insertion of the cassette, the polarity of magnetisation is sensed and provided the polarity is correct printing is enabled. If the polarity is incorrect printing is disabled. When the end of the ink ribbon is sensed, the electromagnet (25; 34) is energised to reverse the polarity of magnetisation of the element (20) so that when the cassette is removed and re-inserted with reversed orientation, the correct polarity of magnetisation is sensed and printing is enabled. A counter may be provided to limit the number of permitted reversals of the cassette.