The J Block mounting block provides multiple uses, including the ability to easily and attractively mount light fixtures and receptacles. Manufactured with ultraviolet stabilized color, minimizing the effects of exposure to the elements. Rich textured appearance. Surface dimensions: 7" x 7". Flange dimensions: 8 1/2" x 7 1/2". J channel internal dimension: 3/4" adjusts to 1 1/4". Knockouts: Round 4" diameter, rectangle 2 3/8" x 3 3/4".