Are rules concerning music hard and fast? Perhaps, but then again, perhaps not. The Musical Ruler will certainly get you thinking about those rules of music, even as you make attempts on a math problem. Each $13.99 purchase comes with the ruler itself and an extensive guide and songbook. The ruler can be played by vibrating and manipulating it’s length at different keys, which are all easily identifiable on the Ruler’s face with precise detail. No idea on what note it is supposed to reverberate when you give your kid a spanking with this!