 It defines three terms "p", "q" and "r" to be in proportion when "p"∶"q"∷"q"∶"r".This is extended to 4 terms "p", "q", "r" and "s" as "p"∶"q"∷"q"∶"r"∷"r"∶"s", and so on.Sequences that have the property that the ratios of consecutive terms are equal are called geometric progressions.