The supporting element is slidably disposed between {HEAD ~ the first rail element} and the second rail element along {TAIL ~ the rail} 
The supporting element is slidably disposed between { HEAD ~ the first rail element } and the second rail element along { TAIL ~ the rail }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL