A simple numerical model of extension in icy satellite shells is developed. Thinning of the ice weakens the shell, promoting further extension. If lateral flow in the lower part of the shell is unimportant, extension is opposed and wide rifts are generated; if lateral flow is rapid, localized extension is favored and narrow rifts are produced. Thick shells or high strain rates favor the development of narrow rifts; low strain rates favor wide rifting. It is proposed that bands, extensional features on Europa, are narrow rifts, while groove lanes on Ganymede are wide rifts. The existence of wide rifting on Ganymede is consistent with previous estimates of a conductive shell thickness at the time of rifting of 4--8 km and a strain rate of 10-15 s-1) strain rates at the time of rifting. Whether this shell thickness applies to present-day Europa depends on the age of band formation, which is poorly known. The difference between rifting behavior on Ganymede and Europa is due to either higher strain rates or higher shell thicknesses on Europa during rifting. The mean stresses required to cause the observed rifting are ≈0.2 MPa for Ganymede and ≈0.3 MPa for Europa. These values are comparable to estimates derived from flexural features.