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Published online by Cambridge University Press: 14 August 2015
The structure of the HR diagram in the G-K giant region is complex, because of the funneling effect and of the intersection of evolutionary tracks for different stellar masses. Therefore it is impossible to derive the age of a cool giant from its location in the HR diagram alone. To remove this indeterminacy a third dimension is needed, and we suggest microturbulence as the appropriate observational parameter.