The symphony now moved forward on much the same lines as those of the piano sonata, and what has been said about the early steps in the development of that form will apply to this one. It should be noted, however, that the introduction of playing opera overtures at concerts greatly aided the development of the symphony. The introduction of this custom was due to the time-honored habit of going late to the opera. The bustle of arrivals prevented the overtures from being heard, and so it became the custom to play them separately. The early instrumental concertos had very great influence on the development of the symphony, because they showed composers the essential differences between piano and orchestral composition. These were not like our modern concertos, written to display the resources of some solo instrument, but were literally concerts of instruments. In the earliest forms contrasts of tone and power were obtained by using a single trio or quartet of strings for the principal passages, and bringing in additional strings (called "ripieno" instruments) to enforce the tone in the tuttis. Alessandro Scarlatti wrote concertos of this sort. Sebastian Bach wrote a number of concertos for instruments, and all of them are in the three-movement form based on the Italian overture. Handel also wrote concertos. But these concertos of Handel and Bach were in the contrapuntal style, and the genius of the sonata form was tending always toward the monophonic style. For that reason these concertos did not have so direct an influence on the symphony as did the overture, which naturally followed the vocal style of the opera.