This 'fluttering' airflow creates high and low pressure waves within the pipe's air column. A high and a low pressure wave form a single "cycle" of the pipe's tone. (See Wind Instrument.) Flue pipes generally belong to one of three tonal families: flutes, diapasons (or principals), and strings. The basic "foundation" (from the French term fonds) sound of an organ is composed of varying combinations of these three tonal groups, depending upon the particular organ and the literature being played. The different sounds of these tonal families of pipes arise from their individual construction. The tone of a flue pipe is affected by the size and shape of the pipes as well as the material out of which it is made. A pipe with a wide diameter will tend to produce a flute tone, a pipe with a medium diameter a diapason tone, and a pipe with a narrow diameter a string tone. A large diameter pipe will favor the fundamental tone and restrict high frequency harmonics, while a narrower diameter favors the high harmonics and suppresses the fundamental. The science of measuring and deciding upon pipe diameters is referred to as pipe scaling, and the resulting measurements are referred to as the scale of the pipe. Reed pipes The sound of a reed pipe is produced by a beating reed: wind is directed towards a curved piece of brass (the reed). A partial vacuum is created by higher velocity air flowing under the reed which causes it to be pulled closed against a hard surface called the shallot. This shuts off the vacuum and allows the reed to spring open again. A tuned resonator extends above this assembly and reinforces the sound produced. The principle is the same as that of the orchestral clarinet. The pitch of a reed pipe is determined primarily by the length of the reed but the volume of air in the resonator supports that frequency. Most reed pipes have a slide to adjust the vibrating length of the reed to fine-tune it. Because of the precision required in the making of the vibrating reed, resonator pipe and its accompanying parts, reed pipes are more complicated to manufacture than flue pipes. By altering any of several parameters (including the shape and volume of the resonator, as well as the thickness and shape of the reed), a reed pipe can produce a wide variety of tonal