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