Ribbon mounting bars are elongated metal or plastic strips with clutch backs that regulation ribbons can be slid or snapped on to. These mounting bars allow for ribbons to be properly and securely displayed in evenly-spaced rows. For the arrangement and proper display for all awards and decorations, please refer to one of the following publications:

US Army: Wear and Appearance of Army Uniforms and Insignia (AR 670-1)
US Navy: Navy and Marine Corpse Awards Manual (SECNAVINST 1650.1G)
US Marine Corps: Navy and Marine Corps Awards Manual (SECNAVINST 1650.1G), MCO 1650.19, MCO P1020.34
US Air Force: Dress and Personal Appearance of Air Force Personnel (AFI 36-2903)
US Coast Guard: Coast Guard Medals and Awards Manual (CMDTINST M1650.25)

US Army Multi-Ribbon Bar 18S R-L