A trill is a quick alternation between the main note and the note above it. It was also known as a "shake". Usually, if the music was written before 1800 the trill is played by starting a note above the written note. If the music was written after 1800 then the trill is usually played by starting on the note written and going up to the note above. This was not, of course, a firm rule: changes in performance style happened gradually. 