A wearable device including at least one sensor configured to sense a physiological parameter of a user. The wearable device including a base housing and a removable housing attachable to the base housing. The base housing and the removable housing portions each including a battery and an electronic subsystem in communication with each other. The battery of the removable housing portion charges the battery of the base housing portion when the removable housing portion is attached to the base housing portion. A second embodiment includes two or more fitness trackers each having an enclosure. The enclosures of the two or more fitness trackers having complimentary shapes that form a unified enclosure when the enclosures are placed adjacently. In a third embodiment, a wearable device includes a first screen display and a second screen display. The second screen display is transparent in at least one operational mode of the wearable device.