system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: vitamin letters and numbers like B6 B12.
gpt
The B vitamins are a set of vitamins that were originally all thought to be a single vitamin (in early attempts to separate the vitamins, they tended to separate under the same conditions) and were then later discovered to be chemically distinct molecules.

The vitamins were initially numbered in the order in which they were characterized. A number of those compounds were ultimately determined not be vitamins at all -- for example vitamin B4 was determined to be the nucleotide adenine and thus there is no vitamin B4.

Vitamins got their letters by being sorted into groups based on their chemical activity. The A vitamins, for example, are characteristically unsaturated hydrocarbons; the B vitamins are enzyme co-factors, etc.