A highly accurate expression for the Q-factor of a direct-coupled filter is derived which is analogous to that for an associated quarterwave-coupled filter. Existing theories usually consider filters of either the maximally-flat or the Cheby̅shev type, and it is shown directly, by transmission-line methods, that these are special cases of a generalized symmetrical filter. Design formulae are given for the bandwidth, pass-band tolerance, and the attenuation in the rejection band both for quarter-wave-coupled and direct-coupled generalized filters.