Tommee Tippee has designed a two tier cassette system to handle every nappy your baby produces. Just as you change nappy sizes or baby foods as baby gets older, so you need to use a different cassette. When your baby starts on solid foods, nappy smells get much stronger and you will need a better odour protection. The cassette for babies over 4 months is specially formulated to stop unpleasant nappy smells. The new cassettes fit all wide-neck Nappy Wrapper Tubs.