What is the difference between impromptu and extemporaneous speech?

When you speak extemporaneously you are literally making up the words of your speech as you go. That does not mean that you do not do preparation. Rather, as you rehearse you work from an outline or speaker notes that remind you of the progression of ideas in your speech. The impromptu speech occurs with little or no time for preparation. There are reduced chances for analyzing the audience, and therefore organizing and encoding requires a fast-thinking speaker. Some communication experts say that an impromptu speech is delivered without any preparation or on the spur of the moment and an extemporaneous speech is given without any notes, but some thought is given to the topic.