Geminiani's significance today is largely due to his 1751 treatise "Art of Playing on the Violin" Op. 9, published in London, which is the best known summation of the 18th-century Italian method of violin playing and is an invaluable source for the study of late Baroque performance practice. The book is in the form of 24 exercises accompanied by a relatively short but extremely informative section of text, giving detailed instructions on articulation, trills and other ornaments, shifting between positions, and other aspects of left- and right-hand violin technique. The instructions in this treatise are famously opposed to those expressed by Leopold Mozart in his "Treatise on the Fundamental Principles of Violin Playing" (1756) on several issues, including on bow hold, use of vibrato, and the so-called "rule of the down-bow", which states that the first beat of every bar must be played with a down-stroke.