The paper analyzes the apparition of the notion of "form" and of neighbour notions such as "motive", "phrase" and "period" in the musical thought of the period between 1790 and 1845. The theoreticians considered include mainly (but not exclusively) H. Chr. Koch, A. Reicha, A. B. Marx, J.-J. de Momigny and J. Chr. Lobe, as well as G. Weber, C. Czerny and S. Sechter. The notion of form stems from several models: periodic, hierarchic or evolutive. The neighbour notions of motive, phrase, sentence (proposition) and period, occasionally found from 1755 onwards, progressively become stabilised in the early 19th century. They describe an arborescent structure formed of integrated units.