A harp is a stringed musical instrument that consists of a set of vertical strings, ranging from longer, thicker strings at the base (low notes) to smaller, thinner (high notes) at the top by the soundboard. It encompasses a wide range of notes, typical harps containing 24 to evcen 40 + strings, containing the entire musical scale. Harps can be small and portable, such as the lighter and less expensive lap harp, or larger and more complex, such as the grand concert harp. Generally, harps with greater amplification have larger soundboards and are more expensive. Harps are a unique form of stringed instrument, often incorrectly compared to a vertical piano, and have existed before the piano was even created. They typically have levers located above strings to adjust from sharp to flat to normal, and some of the larger harps have pedals. Improvisation is a key part of the harpist's world. Sheet music created specifically for the harp can be purchased.