The approach utilized in the design of achromatic periods formed of two layers a high-index and a low-index is developed by combining the first two periods together. These two periods are therefore reduced to three layers a central one, an upper layer and a lower layer. Both the upper and lower layers are matched to the central at two different wavelengths. This forms the so called a combined achromatic period or the basic unit. Multilayers showing low transmittance bands are then synthesized of this basic unit. Parameters used in the synthesis of such multilayers are pointed out and their control role is investigated.