What is a monochord?
According to Wikipedia: A monochord is an ancient musical and scientific laboratory instrument. The word "monochord" comes from the Greek and means literally "one string." A misconception of the term lies within its name. Often a monochord has more than one string, most of the time two, one open string and a second string with a movable bridge. In a basic monochord, a single string is stretched over a sound box. The string is fixed at both ends while one or many movable bridges are manipulated to demonstrate mathematical relationships between sounds.
Here's a "how to video":