Coherence
The second quality a finished composition should have is coherence. If you know what cohere and cohesion mean (perhaps you have met these words in science study) you have the germ of the term's meaning. It means "stick-together-itive-ness." The parts of a speech should be so interrelated that every part leads up to all that follows. Likewise every part develops naturally from all that goes before, as well as what immediately precedes.
There must be a continuity running straight through the material from start to finish. Parts should be placed where they fit best. Each portion should be so placed—at least, in thought—that all before leads naturally and consistently up to it, and it carries on the thread to whatever follows. This prevents rude breaks in the development of thought. Skilfully done, it aids the hearer to remember, because so easily did the thought in the speech move from one point to another, that he can carry the line of its progression with him long after. So the attainment of coherence in a speech contributes directly to that desired end—a deep impression.
Incoherent speeches are so mainly because of absence of plan, whether they be short or long, conversational or formal.