A touch sensor on a device is configured to accept user input in the form of touches. Control areas on the touch sensor may be designated to correspond to keys on a keyboard or other commands. These control areas may be arranged on either side of the touch sensor to allow the user to activate control areas with their thumbs while holding the device. Size and spacing of the keys may vary dynamically in response to environmental conditions, user interactions, and so forth. Thus, the area of a control on the touch sensor may be larger when operated in a high vibration or motion environment, such as on a train, walking, and so forth. Additionally or alternatively, the position of control areas may dynamically change to adjust to the user.