Capacitive touch sensing is increasingly used in musical con-trollers, particularly those based on multi-touch screen interfaces. However, in contrast to the venerable piano-style keyboard, touch screen controllers lack the tactile feedback many performers find crucial. This paper presents an augmentation system for acoustic and electronic keyboards in which multi-touch capacitive sensors are added to the surface of each key. Each key records the position of fingers on the surface, and by combining this data with MIDI note onsets and aftertouch from the host keyboard, the system functions as a multidimensional polyphonic controller for a wide variety of synthesis software. The paper will discuss general capacitive touch sensor design, keyboard-specific implementation strategies, and the development of a flexible mapping engine using OSC and MIDI.