1. L. Euler, Elements of Algebra, Chapter I, Article I, #3. Third ed., Longman, Hurst, Rees, Orme and Co., London England, 1822. “Now, we cannot measure or determine any quantity, except by considering some other quantity of the same kind as known, and pointing out their mutual relation.”
2. D. H. Krantz, et al, Foundations of Measurement, Academic Press, NYC, NY, 1971, Vol. 1, page 3, 1.1.2, Counting of Units. This three volume work is considered the basis of representational measurement theory.
3. K. Krechmer, Relative measurement theory (RMT), Measurement, 116 (2018), pp. 77-82.
4. International Vocabulary of Metrology (VIM), third ed., BIPM JCGM 200:2012, quantity 1.1. 03 December 2022.
5. K. Krechmer, Relative measurement theory (RMT), The verification is in the Appendix of RMT.