For at least some children (for my first two years at school I was one of them) it is not obvious what particular-shaped squiggles on a page might have to do with words. It is easy to miss the three-way idea (as I did) that the squiggles represent sounds, the sounds make up the words, and the sounds are in an ordered sequence.
A lost insight from antiquity
About 2,600 years ago, someone in what is now Greece or Italy seems to have had the clever and reasonable idea of making the shapes of letters fit the sounds they represented. (There were only capital letters at the time.)
By this idea, for the sounds which are said through the nose, M and N, there are points at the top and downward facing openings at the bottom, two where the mouth is closed by the lips and one where it is closed by the tongue tip. And for the sounds with a complete closure, B and D, there are rightwards facing arcs and a bar closure on the left, two arcs where the closure is by the lips, and one where the closure is by the tongue tip. By this idea, speech sounds, or what are now known as ‘phonemes’, have internal constituents, or what are known as ‘features‘.
Whoever had this idea would seem to have adopted slightly different ways of writing D and B from different Greek cities to bring out the modern contrast. I show the idea in the matrix above.
2,600 years later we still use this design principle for these four capital letters. The idea is simple, elegant, graphical, easily-understood, and appropriate. But for some unknown reason, it was only applied to the four letters. Possibly that ancient scholar was Etruscan and the cleverness was not appreciated by the conquering Romans. Or the scholar may have died before being able to explain the idea to contemporaries.
This cleverness was 2,000 years ahead of its time. It was not developed any further until the work of William Holder and Alexander Melville Bell, and Noam Chomsky and Morris Halle in the modern era. But M, N, B and D are a natural point of departure – where the fundamental principle of speech sounds having these and other features is most easily detectable – not that it was ever easy.