The story goes this way:...............
1) An alphabetic arrangement would overwork some fingers and underwork others, resulting in typing being more tiring. The QWERTY arrangement puts the most-commonly used letters by the strongest fingers. Notice how spread out the most common letters are? Look at 'etaion' on a QWERTY keyboard.
2) An alphabetic arrangement would use the same hand too many times in a row, preventing you from setting up one hand while the other is typing. It's much faster to type two letters on different hands than two letters on the same hand. The QWERTY arrangement takes the most common two-letter sequences and puts one letter on one hand and one on the other. (For example 'an', 'le', 'ie', and so on.)
3) An alphabetic arrangement would use the same finger twice in a row too often, wasting time as you move a finger rather than setting up one finger while another is typing. The QWERTY arrangement breaks up even less common letter sequences so they're not on the same finger. ('nw', 'ce', and so on.)
4) An alphabetic arrangement would require more vertical motion of the fingers. For example, if you look at a QWERTY arrangement, you'll see that the hardest to type sequences due to vertical motion are sequences almost never used in English. (For example, 'wx', 'oq', 'sw', and so on.)
