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1. In writing triads for four voices, there should be no more than an octave between Soprano and Alto or Alto and Tenor.
Close structure of a triad refers to the arrangement of Soprano, Alto, and Tenor so that there is no more than an octave between Soprano and Tenor. Open structure means the arrangement of these three upper voices have more than an octave between Soprano and Tenor.
Taken from- http://members.nbci.com/tgue1451/checkp.html