The passage can be a closed slot, open notch, or {TAIL ~ other structure} on which {HEAD ~ the belt} travels 
on travels
The passage can be a closed slot , open notch , or { TAIL ~ other structure } on which { HEAD ~ the belt } travels
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL REL OTH HEAD HEAD HEAD HEAD HEAD HEAD REL