Many people find the convenience of wearing CORE on the arm strap (compared to a chest strap) an acceptable trade-off. Please note, when CORE is worn on the arm, the accuracy of the core body temperature measurements is lower as the thermoregulation of the arm can different from the torso/chest. When it is more convenient to use an arm strap and the level of accuracy from wearing CORE on the torso is not needed, the arm strap is made of a comfortable elastic material, perforated for improved breathability and with velcro for convenience and adjustability. The black arm strap is 'one size fits all' (as it can be adjusted to fit) and is designed to remain comfortable for sports and all-day use. This includes ONLY the arm strap and the CORE device is not included and is shown in the photos for demonstration.