There are two types of liner, one piece and two pcs.
Two pcs liners are mounted in stainless or Polycarbonate shell. Then a short milk tube is connected to liner through a polycarbonated small part, called Sight glass makes the liner's height in a standard length for milking and attaching to udder.
This short tube is in Silicon or rubber with 16 or 18 cm length.
Technically we call this tube "Short Milk Tube" or SMT and usually it must be replaced with new one every time milking liner is replaced.
Being flexile and high quality are subjects which must be observed in SMT production. As it is the part where is under vacuum and stress.