Models for granitic melt viscosities reveal the influences of H2O, temperature and tectonics on melt segregation, ascent and emplacement. Low-T magmas are wetter than those formed at high-T, so log10 melt viscosity (Pa s) is 3.2–6.3 for leucogranite to tonalite, only 10–1000 times more viscous than basalts. There should be no correlation between critical dyke width (to prevent freezing) and tectonic setting. Granitic magma ascent rates in dykes vary by <10%, and melt extraction rates should be similar in different settings. Emplacement styles are not predetermined by composition or temperature, but by local structures and tectonics.