The coupling parts 28 29 are preferably also realized in {HEAD ~ the shape} of a tongue 31 and {TAIL ~ a groove} 32 
of
The coupling parts 28 29 are preferably also realized in { HEAD ~ the shape } of a tongue 31 and { TAIL ~ a groove } 32
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH