The optimum size, characteristic time, necessary power and energy for magnets for a semi-continuous high field above 40 tesla are strongly dependent on the type of coil-system; several different types can be distinguished. The increase in temperature of the coil is, for a homogeneous coil, about proportional to the square of the maximum field, and nearly independent of all other parameters. The temperature rise and minimum power needed can be reduced considerably by using a two-stage coil-system or a coil system with independently powered sections. Some general properties of these latter types of coil systems will be discussed.