Background Trimming (Oval) Description
An black oval-shaped embroidered item 1 3/8 inches (3.49 cm) in height and 2 1/4 inches (5.72 cm) in width overall, edged with a 3/8 inch (.95 cm) yellow border.

History
The background trimming was originally approved for the 2d Battalion, 509th Infantry Regiment on 5 June 1963.  It was amended to change the dimension of the black border on 16 January 1964.  It was revised to change the description of the design on 20 February 1991.  The trimming was redesignated for the 509th Infantry Regiment and amended to change the color of the border on 23 June 1992.

Source: The US Army Institute of Heraldry