Orchestration is the art of writing music for a possibly large ensemble of instruments, by blending or opposing their sounds and grouping them into an orchestral texture. We aim here at providing a deeper understanding of orchestration in classical and early-romantic symphonies by analyzing, at the bar level, how the instruments of the orchestra organize into melodic, rhythmic, harmonic, and mixed layers. We formalize the description of such layers and release an open corpus with more than 7900 annotations in 24 first movements of Haydn, Mozart, and Beethoven symphonies. Initial analyses of this corpus confirm specific roles of the instruments and their families (woodwinds, brass, and strings), some evolution between composers, as well as the contribution of orchestral texture to form. The model and the corpus offer perspectives for empirical and computational studies on orchestral music.