We have 16mm and 18mm T-Moulding. You can use 18mm T-Moulding on 16mm material and it looks fine, 1mm over have either side. using 18mm T-Moulding on 18mm material just means you need to be very careful that your slot is exactly in the centre! A 1.6 - 2mm T slot cutter router bit is best for making the slot.