Rigorous formulations closely following current practice, are proposed for the absolute and the relative estimation of certain target quantities. A system of standards is defined as a set of estimates that has certain properties. This is followed by a discussion of the meaning of “reducing estimates to a specified system“ and certain procedures are suggested for making this process less ambiguous. It is finally shown that relative estimates can be employed for replacing a system which is based on absolute measurements alone by another, more accurate system.