elem.6.def.1
Similar rectilineal figures are such as have their angles severally equal and the sides about the equal angles proportional.
Ὅμοια σχήματα εὐθύγραμμά ἐστιν, ὅσα τάς τε γωνίας ἴσας ἔχει κατὰ μίαν καὶ τὰς περὶ τὰς ἴσας γωνίας πλευρὰς ἀνάλογον. &`2 1$