Klopfzeichen is the first full-length album by German experimental music trio Kluster.Klopfzeichen was recorded on December 21, 1969 at Rhenus-Studio, Gordorf, Germany. Liner notes on the CD reissue on the Hypnotic label as well as some websites place the recording date precisely one year later in 1970. This is incorrect as it would place the recording after the initial release date.