The importance of micromixers for microreaction technology demands some well-founded knowledge on the mixing behaviour of these devices. We show by numerical simulations as well as experimentally that increasing vorticity inside static T-shaped micromixers with rectangular cross-sections occurs even at low Reynolds numbers and that these effects can be used to improve the mixing quality. Hereby we distinguish between three flow regimes. Additionally, we give a new identification number for this kind of problems and we show that this number suits for static T-mixers. This is confirmed by simulations and experimental measurements.