The economical Atlas Sound 121-18 cabinet is 22 5/ 16" wide, 18 1/ 2" deep, and contains 21RU vertical panel spaces. The 121-18 cabinet includes one pair of 11 ga. CRS fixed mounting rails. The serie is constructed of 16 ga. CRS and include base, top panel, side panels, mounting rails, a removable rear door with cylinder lock and flush door pull handle. Offers several installer-friendly advantages, including an expanded 6RU opening in the cabinet top and bottom which can be left open for wire access and/ or ventilation, or closed using a standard vent or solid panels. Rear doors (included with all models) are hung with quick-release spring hinges which allow them to be rotated 180Â° for right or left-hand opening. If a front door is ordered, it will be keyed differently than the rear door for security. Rack rails are precision tapped 10-32 and recessed 11/ 16" (17.5mm). These plated rails include convenient RU indicator hash marks to aid in equipment alignment. Rails are standard E.I.A. 5/ 8"-5/ 8"-1/ 2" spacing.