Sentence:  The severe yet harmonious vases of the previous fifty years , the Strong Geometric style of the late ninth century , display as firm a mastery of the <WSD>principles</WSD> underlying Geometric pottery ; but artists now were ready to refine and elaborate their inheritance . 
Meaning of the word marked with <WSD> tags:  a basic generalization that is accepted as true and that can be used as a basis for reasoning or conduct