conventional cell就是14种布拉维格子的另一个别名。

reduced cell是属于14种布拉维格子种的P格子，而且需要满足两个条件：1）最短的晶轴；2）a <= b <= c，and all angle either < 90 or >= 90。

both cell setting have the same Niggli cell setting, therefore, they are not polymorph （see figure below）.