Two concepts new in square bandpass design make the design process easier and more systematic. One is the use of adjacent high-order rejection bands. The second is the use of effective index ratio. This paper extends the work of L. I. Epstein [J. Opt. Soc. Am. 42 806 (1952): "The Design of Optical Filters"] and A. Thelen [J. Opt. Soc. Am. 50 1533 (1966): "Equivalent Layers in Multilayer Filters"].