When ‘vitamin A’ was discovered, the next active substance was termed ‘vitamin B’. Later it transpired that ‘vitamin B’ is not actually a single substance, but a group (‘complex’) of different vitamins. As the following letters were already assigned, the various substances were given numbers, resulting in the names vitamin B1, B2, B6 and B12. Other B vitamins were discovered later and were given their own names (e.g., folic acid). The gaps in the numbering are due to the fact that a number of substances initially mistaken for vitamins have been removed from the complex.