Analysis of information sources in references of the Wikipedia article "1988–89 NHL season" in English language version.
The NHL has 60 referees and linesmen under contract and among them are 11 men who do not wear helmets. This is allowed through a grandfather clause in the collective agreement between the NHL Officials' Association and the league, which made wearing helmets mandatory beginning with the 1988–89 season. However, just as the NHL did with its players when helmets became compulsory for them in 1979, a grandfather clause was inserted in the agreement. All referees and linesmen who were employed on or before Sept. 1, 1988 did not have to wear a helmet.
