Attachment systems for attaching a wearable electronic device to a user including a sizing band and a retaining band are disclosed. The sizing band includes at least one sizing eyelet and a band-insertable end. The retaining band includes at least a post for insertion into the sizing eyelet of the sizing band and a concealment aperture configured to receive the band-insertable end of the sizing band. On a bottom surface of the retaining band can be a recessed guide bed configured to guide the band-insertable end along the bottom surface of the retaining band. A wearable electronic device can couple to both the sizing band and the retaining band by accepting an end portion of each band into receiving channels extending into opposite sidewalls of the housing.