Abstract: A musical slide rule having a slide rule body in the form of a flattened sleeve having two flat surfaces defining a front and back side at two substantially parallel edges and having two open opposing ends. The front and back sides have spatially arranged (horizontally and vertically) apertures and music data printed thereby for displaying cooperative music data which is printed on a flat slide which is inserted through and between the front and back side of the slide rule body at one of the two opposing ends. The slide is operatively positioned by the user to display a correlation between the music data of the slide (showing through the apertures of the slide rule body) and the music data of the rule body.