Euclid's Elements
Book VI
DEFINITIONS

1. 
Similar rectilineal figures are such as have their angles severally equal and the sides about the equal angles proportional. 
[2. 
Reciprocally related figures. This definition is neither used by Euclid nor translated by Heath.] 
3. 
A straight line is said to have been cut in extreme and mean ratio when, as the whole line is to the greater segment, so is the greater to the less. 
4. 
The height of any figure is the perpendicular drawn from the vertex to the base. 
Book VI Propositions
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