We present a technical lemma that establishes, in a general framework, an interval-excitation property using a specific class of impulse sequences. According to such an interval-excitation property, an impulse applied within a time interval generates excitation within the same interval. This property does not hold for arbitrary exciting sequences. The lemma is a unified statement of previous results corresponding to particular choices of the exciting sequence. (C) 2002 Elsevier Science Ltd. All rights reserved.