« Strictly speaking, interchangeable assembly consists in making the different parts of a mechanism so uniform in size and contour that each part of a certain model will fit any mating part of the same model. However in ‘selective assembly’ the parts are sorted according to size and type and assembled or interchanged with little or no effort. The chief purpose of ‘selective assembly’ is the production of large quantities of duplicate parts as economically as possible. »
-John M. Amiss « Machinery Handbook Section 13: Tolerances and Allowances »

“Within a selected key the players will each respond in succession to the other’s melody. For a determined set of bars one player presents a melody after which he may pause allowing a second player to respond and embellish. Following the second players response the original player will resume with a response to the second player and so on. One finds this pattern to have been transmitted over the centuries in various forms of cultural expression—in religious observance; public gatherings; sporting events; even in children’s rhymes; »
-John Fallow « Sub-Saharan Music and Cultures »