Hi David! There are four versions of the Glycerin 20, as there is a choice between Classic and StealthFit and a choice between Neutral or GTS. The Classic and StealthFit versions will refer to how the upper material fits. The StealthFit will have more of a socklike feel with less adjustability in the lacing, while the Classic is more of that traditional athletic shoe fit. Neutral and GTS refer to the support in the shoe. The GTS version has our GuideRail support system, which is intended to help support runners who overpronate (feet/ankles roll inward). We recommend the neutral model if you do not overpronate.