High Quality Content by WIKIPEDIA articles! An unusual unit of measurement is a unit of measurement that does not form part of a coherent system of measurement; especially in that its exact quantity may not be well known or that it may be an inconvenient multiple or fraction of base units in such systems. This definition is deliberately not exact since it might seem to encompass units such as the week or the light-year which are quite "usual", in the sense they are often used; if they are used out of context they may be "unusual", as demonstrated by the FFF system of units.