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Hrimaly/Mogill is a beginner to intermediate scale book, starting with one octave scales and going up to three octaves, with relatively simple exercises to add to the basic scales.
Flesch and Galamian are the two most commonly used intermediate-to-advanced scale books for violin and viola. Now that I actually look at them (I usually just practice Galamian-style three octave scales and basic arpeggios without the books), I realized that both are more accessible in some ways and less accessible in others, so I wouldn't necessarily say Flesch is easier as I implied before. The two sometimes use different fingerings, and differ somewhat in the exercises they include. The biggest differences between the books:
* Galamian includes 2-octave scales and one-position exercises, while Flesch does not, so it can be used without having started playing three-octave scales. (This was an error on my part earlier -- I thought it was Flesch rather than Galamian that included 2-octave scales.)
* Flesch writes out everything in standard notation and organizes things by key (all the exercises placed together under each key), whereas Galamian has only note heads and organizes things by exercise (all the keys under each exercise). This can make Galamian difficult to read when you start practicing exercises beyond just the basic scales.
* On the other hand, Galamian includes an insert with a one-stop list of bowing and rhythm variations, while Flesch just prints each scale using an example of a bowing or rhythmic variation and assumes that you or your teacher will come up with other variations to practice.
* The thing that distinguishes Galamian more than anything else is the practice of "acceleration scales" where you start with one note to a bow played very slowly, then play the same scale with two notes to a bow, and so on. In theory, this can continue until you are playing the entire scale in one bow. (Galamian adds turns to the beginning and end of scales to make the number of notes divisible by as many smaller numbers as possible, 32 notes for 2 octave scales and 48 notes for 3 octave scales.)