This paper proposes a fundamentally new unknown input observer (UIO) which has, uniquely, r decoupled modes, provides r linearly independent combinations of plant system states, and has zero gain to all p unknown inputs, The parameter r is adjustable, If the plant system satisfies the conditions of the existing UIO, then r = n = plant system order, and there is no difference between our UIO and the existing regular UIO. However, if the plant system satisfies either of the following two significantly more general conditions : m > p (m is the number of plant output measurements) and at least one stable transmission zero, then r < n and our UIO can still generate some constrained state-feedback control signals, Hence, this new UIO can completely unify the regular UIO (r = n) and the static output feedback (r = nz)-the two basic feedback structures of modern control theory far from being unified before.