Arranger is a project on automatic instrumentation. In a nutshell, we aim to dynamically assign a proper instrument for each note in solo music. Such an automatic instrumentation model could empower a musician to play multiple instruments on a keyboard at the same time. It could also assist a composer in suggesting proper instrumentation for a solo piece. Our proposed models outperform various baseline models and are able to produce alternative convincing instrumentations for existing arrangements. Check out our demo!