Why do I think about these things?

Alright... hear me out.

  The word we use to define a set of runes associated with specific sounds is "alphabet". However it is common that people should abbreviate this word to simply the ABCs; those being the first three letters of our alphabet. This word stems from two root words, alpha and beta; these being the equivalent to "a" and "b".
  With this knowledge we now have to ask the question... Why do we call it the ABCs when originally it was just the ABs?