Abstract: We give a unified discussion of two different approaches to the design of grazing incidence monochromators with a variable line spacing (VLS) grating for soft X-ray undulator sources. Neither one uses an entrance slit and both work with a fixed position exit slit. In one approach, being constructed at LNLS and designed for the energy range 100 < E < 1000 eV, the choice of the VLS parameters allows for the operation at a variable c-value with a single plane grating and little sacrifice in the maximum resolving power. In this case source size limited resolving power of circa 40 000 is expected at 100 eV. In the second approach, for the storage ring in Wisconsin, two gratings covering the energy range 40 < E < 1500 eV are used, one capable of delivering a resolving power larger than 20 000 at 860 eV and the other lower resolving power but much higher flux Ver menos