[NTG-context] what does rscale=auto achieve?

Dear list, I have the following sample: \definefallbackfamily [mainface] [rm] [DejaVu Serif] [preset=range:cyrillic, rscale=auto] \definefallbackfamily [mainface] [rm] [GFS Didot] [preset=range:greek, rscale=auto] \definefontfamily [mainface] [rm] [TeX Gyre Termes] \setupbodyfont[mainface] \starttext \startTEXpage[offset=1em] аaα \stopTEXpage \stoptext I thought that "rscale=auto" would scale the typeface so that its x-height would match the x-height from the main typeface. Since I don’t see any scaling, could anyone explain me what I’m missing? Many thanks for your help, Pablo -- http://www.ousia.tk