The $599 special is on the Simmons Smart Motion Adjustable base 1.0 in the Twin X-long. Two of these would make a king.
The SmartMotion Base 1.0 from Beautyrest is an advanced new sleep optimization system with highly accurate multistage sleep tracking. It provides you with a rejuvenating sleep experience. Benefits include adjustable head and foot angles, mattLock mattress retention system, This Base lets you find your most comfortable position for a better, more refreshed you. The Integrated sensors capture precise readings of awake time, light sleep, deep sleep and REM sleep throughout the night, uncovering the full depth of your sleep patterns and ways to improve them. Enjoy added USB & Power Outlets, for more freedom with your accessories such as cell phone & tablets.
Queen is being shown in the picture.
Two Twin X-Long’s make a King
All other sizes will be the Simmons Beautyrest Advanced Motion Base