Becoming a practical musician traditionally requires an extensive amount of preparatory work to master the technical and theoretical challenges of the particular instrument and musical style before being able to devote oneself to musical expression. In particular, in jazz improvisation, one of the major barriers is the mastery and appropriate selection of scales from a wide range, according to harmonic context and style. In this paper, we present AutoScale, an interactive software for making jazz improvisation more accessible by lifting the burden of scale selection from the musician while still allowing full controllability if desired. This is realized by implementing a MIDI effect that dynamically maps the desired scales onto a standardized layout. Scale selection can be pre-programmed, automated based on algorithmic lead sheet analysis, or interactively adapted. We discuss the music-theoretical foundations underlying our approach, the design choices taken for building an intuitive user interface, and provide implementations as VST plugin and web applications for use with a Launchpad or traditional MIDI keyboard.