音数为3又2分之1的的五度音程称之为纯五度，如C-G，D-A，E-B，bB-F，B-F#，F#-C#，bC-bG等就是纯五度。
同理，加大三度的根音，其上加一个纯五度。
This is the same as a root with a major third and a perfect fifth added above it.
如在多声部音乐萌芽时期除了应用八度音程以外，人们只采用频率比数最简单的，即音响最协和的纯五度与纯四度音程；
Such as at the beginning periods of several-department music, people always adopt to the the most simple frequency or the purest perfcet fourth and fifth interval.
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