A discussion with nakurusil left open some questions to which answers are highly appreciated. 1. We measure in physics periods and reckon frequencies or vice-versa. 2. Is it possible to teach the Doppler Effect without involving the concept of wave crest? 3. When an Author says that the observer involved in a Doppler Effect receives two successive wave crests remmaining located at the same point, does he make the assumption that the period is "very small"? I think that the correct answers are usefull for all of us. In my oppinion in the case 1 the first variant is correct as long as the observer uses his wrist watch, in the second case the concept of wave crest is usefull whereas in the third case the very small (locality) assumption is made.