This is equally true of strings; if stretched between two posts and bowed, the sound given out by a string is feeble, but if stretched over a sounding board, as in the piano, or over a wooden shell, as in the violin, the sound is intensified. Any note of the instrument will force the sounding body to vibrate, thus reenforcing the volume of sound, but some tones, or modes of vibration, do this more easily than others, and while the sounding board or shell always responds, it responds in varying degree. Here again we have not only enrichment of sound but also individuality of instruments. 271. The Kinds of Stringed Instruments. Stringed instruments may be grouped in the following three classes:-- _a_. Instruments in which the strings are set into motion by hammers--piano. _b_. Instruments in which the strings are set into motion by bowing--violin, viola, violoncello, double bass. _c_. Instruments in which the strings are set into motion by plucking--harp, guitar, mandolin. [Illustration: FIG. 186.--1, violin; 2, viola; 3, violoncello; 4, double bass.] _a_. The piano is too well known to need comment. In passing, it may be mentioned that in the construction of the modern concert piano approximately 40,000 separate pieces of material are used. The large number of pieces is due, partly, to the fact that the single string corresponding to any one key is usually replaced by no less than three or four similar strings in order that greater volume of sound may be obtained. The hammer connected to a key strikes four or more strings instead of one, and hence produces a greater volume of tone. _b_. The viola is larger than the violin, has heavier and thicker strings, and is pitched to a lower key; in all other respects the two are similar. The violoncello, because of the length and thickness of its strings, is pitched a whole octave lower than the violin; otherwise it is similar. The unusual length and thickness of the strings of the double bass make it produce very low notes, so that it is ordinarily looked upon as the "bass voice" of the orchestra. _c_. The harp has always been considered one of the most pleasing and perfect of musical instruments. Here the skilled performer has absolutely free scope for his genius, because his fingers can pluck the strings at will and hence regulate the overtones, and his feet can regulate at will the tension, and hence the pitch of the strings. Guitar and mandolin are agreeable instruments for amateurs, but are never used in orchestral music. [Illustration: FIG. 187.--A harp.]