If a number have any part whatever, it will be measured by a number called by the same name as the part.
如果一个数有任意一个部分，那么它将被一个与该部分同名的数所度量。
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正文图形由校订坐标生成；点、线、角、圆可与证明和问答联动。
For let the number A have any part whatever, B, and let C be a number called by the same name as the part B; I say that C measures A.
设数A有任意部分B，且C是与部分B同名的数。
For, since B is a part of A called by the same name as C, and the unit D is also a part of C called by the same name as it, therefore, whatever part the unit D is of the number C, the same part is B of A also; therefore the unit D measures the number C the same number of times that B measures A.
因为B是A的部分且与C同名，而单位D是C的部分且与C同名，所以单位D是C的多少部分，B也是A的多少部分。
Therefore, alternately, the unit D measures the number B the same number of times that C measures A.
因此，单位D度量数C的次数等于B度量A的次数。
由VII.15，交替地，单位D度量数B的次数等于C度量A的次数，故C度量A。