GDM Ghost Mark II full face motorcycle helmet with Bluetooth is DOT FMVSS-218 certified. Its shell is constructed using an advanced lightweight composite poly-alloy. It features five points of ventilation with removable and washable liner and cheek pads. It also includes a double D-ring chin strap and comes with two visors in the box. Please refer to the following size guide to determine your correct size. To choose your correct helmet size, please measure the circumference of your head 1" above your eyebrows around your head and refer to the GDM Duke Helmets size guide below. Please note that the size guide is for reference only and cannot guarantee a perfect fit. SMALL: 21 5/8 – 22 IN 55-56 CM | MEDIUM: 22 ½ -- 22 7/8 IN 57-58 CM | LARGE: 23 1/4 -- 23 5/8 IN 59-60 CM | X-LARGE: 24 -- 24 3/8 IN 61-62 CM | 2X-LARGE: 24 3/4 -- 25 ¼ IN 63-64 CM BUTTON FUNCTIONS POWER Button: -Long press 2 seconds to turn the Bluetooth receiver on. If no available phones are found, the device will go into “waiting mode” for paring and the blue and red LED lights will flash quickly. If an already paired device is found, the devices will automatically connect. To turn the device off, long press 3 seconds and the Bluetooth module will power off. -Long press 1 seconds to start SIRI/Google assistant. -To change the language, double press while device is connected, and beeping sound is heard. MENU Button: -While playing music, short press to play and pause -Device will automatically answer an incoming call in 10 seconds. -long press 1 second to reject an incoming call -Short press to hang up after a call -Short press to answer an incoming call -Long press 3 second to start or stop EM radio -Double press to redial the last outgoing call (+) Button: -Short press for volume increase -Long press for the next track (-) Button: -Short press for volume decrease -Long press for the previous song