Reaktor is a graphical moular software music studio of proprietary license developed by Native Instruments. It lets musicians and engineers design and build their own instruments, samplers, effects and sound design tools. It is supplied with many ready-to-use instruments and effects, from emulations of classic synthesizers to futuristic sound design tools. In addition, more than 2000 free instruments can be downloaded from the growing User Library. One of Reaktor's unique selling points is that all of its instruments can be freely examined, customized or taken apart; Reaktor is a tool that effectively encourages reverse engineering. Reaktor Session is a discontinued, limited version of the software that allowed musicians to play Reaktor files but not edit or reverse-engineer them.