One does not only talk about the length in inches of this sheet of paper but also about the length of this sheet, about length in inches and about length. A clarification of these and related concepts results from a combination of the theory of the length in a definite unit as a fluent, developed by one of the authors, with the other's concept of 2-place fluents. The length ratio L is defined by pairing a number L (α, β) to any two objects of a certain kind, α, β (in a definite order). L thus may be described as the class of all pairs (α, β), L (α, β) for any objects α, β of the said kind. Length of this sheet and length in inches are specializations of this 2-place fluent.