The teaching methods of Johann Sebastion Bach are recounted by his son, Philip Emanuel Bach:
The first thing he did was to teach his pupils his special ways of touching the keyboard. For this he made them practice for months nothing but separate exercises for all the fingers of both hands, with constant attention to the production of a clear tone.Over a period of some months, no pupil was excused these exercises, and according to his firm opinion, they should be continued from six to twelve months at least. But if he found that anyone, after some months of practice, began to lose patience, he was so considerate as to write little pieces in which these exercises were linked together. To this type belong the “Six Little Preludes for Beginners”, and still more the fifteen “Two-Part Inventions”. He wrote both during his teaching, and so attended to the immediate needs of the pupil; afterwards he transformed these pieces into beautiful, expressive little works of art … After this he set his pupils to studying his own greater compositions which, as he well knew, would give them the best means of exercises their powers.
Source: Dowley, Tim (1981) Bach. London: Omnibus Press, p.57.