Other authors seem to differentiate by degrees. Leopold Mozart includes tremolo in chapter 11 of his violin treatise, but describes an unnamed vibrato technique in chapter 5 on tone production. His son, Wolfgang Amadeus Mozart, appears to take the exact opposite definition as his father: in a letter to his father, Mozart criticizes singers for "pulsing" their voice beyond the natural fluctuation of the voice, the latter of which being pleasant should be imitated on the violin, winds, and clavichord (with bebung).