Called an oval for obvious reasons, Airborne background trimming is worn beneath Parachutist and Air Assault badges earned by personnel serving in units that are designated as Airborne by “structure, equipment, and mission. This is why the oval for the 21st Engineer Battalion—originally constituted in 1861 and with battle honors the Civil War, War With Spain, War With Spain, Philippine Insurrection, Mexican Expedition, and both World Wars—was not approved until 14 March 2014: the first time the Battalion was part of an Airborne unit came when it was assigned to the 3rd Brigade Combat Team, 101st Airborne Division and activated on 16 April 2014.
With the main portion of the Parachutist and/or Air Assault Badge (i.e., not including the star or wreathed star of the Senior and Master Parachutist Badges) centered on the oval, the combination is worn on AGSU and ASU coats, Class B ASU shirts, and the green or blue Army Maternity tunic. The two devices count as one when determining maximum badge numbers, and the edge of the oval (and the top of the star / wreathed star if applicable) is used for measuring distances from the badge.
General guidance on the wear of Airborne background trimming is found in the January 2021 edition of DA PAM 670-1,
Chapter 21-32 (b). Additionally, regulations concerning badge and insignia placement vary between the uniforms on which Airborne background trimming is worn. Detailed instructions on badge placement is found in the same edition of DA PAM 670-1,
Chapter 22-16 (b).
https://armypubs.army.mil/epubs/DR_pubs/DR_a/ARN30948-PAM_670-1-000-WEB-1.pdf#page=275
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