Hi Matthias, I've been following the discussion about physical quantities for some times now and I'm wondering if someone has thought of representing base units by prime numbers? Any positive natural number can unambiguously be split into it's prime factors, so any rational number would be an unambiguous representation of a derived dimension with the exponents of the prime factors representing the exponents of the base dimensions. The only problem are common factors in nominator and denominator, which could be normalized using the Euclidean algorithm. Multiplying dimensions is then reduced to multiplying rational numbers with subsequent normalization. That should be a lot easier then dealing with sorted lists. I might find some time to write up an example implementation. Interested? Best wishes, Andreas