I remember reading "The Diving Bell and the Butterfly: a memoir of life in death" by Jean Dominique Bauby. He wrote the book by blinking at each letter as someone recited the letters in ESA sequence vs ABC. Of course since Jean was French the frequency of letters (ESA) is different from English (ETA).
I guess Wheel of Fortune, Scrabble, and NY Times Crossword experts come to know the letter frequency tricks. Even our QWERTY typewriter keyboard was designed with letter frequency in mind. And of course the 80/20 rule applies in letters also - the top 12 letters comprise 80% of the usage.
I'm not sure how we landed on 26 letters. And what about all the other symbols?
I'm particularly partial to the space. :)