cbf94194_Euclid_5_Intro_html__Definitions
[0] 17. [Definitions] 
A ratio through an equal (ex aequali ratio) is, given that there are several magnitudes and others equal to them in number are taken two by two and in the same ratio, whenever in the first group of magnitudes the first is to the last, so in the second group of magnitudes the first is to the last. Or, alternatively, taking the extreme terms by removing the middle terms.
Euclid.5.Intro.html
Definitions