1. A subaltern is an officer below the rank of Captain. The subaltern appointed unit Senior Subaltern will have status immediately following that of the junior Captain though he may not be the senior subaltern by virtue of time in rank.
2. The unit Senior Subaltern has many unwritten and unofficial duties, the principal of which is to act as a link between the subalterns and the commanding officer. He advises subalterns on matters of dress, deportment, military etiquette and Regimental customs, and he corrects minor faults without resorting to formal action. The Senior Subaltern will be guided on all matters by the unit Adjutant.
3. The Senior Subaltern is the only officer in the battalion authorized to conduct a 'Whistle Check' in the Mess.
(Regimental Standing Orders for The Royal Canadian Regiment, article 404, 1992 Revision)
In the days before CADPAT, when the Army wore the olive drab combat uniform, dress for Master Corporals and above in The RCR (including officers) included the wear of an olive drab lanyard with an issued whistle. When some officers developed a habit of considering the whistle unnecessary for their personal use, they might try to replace it with another item (keys, comb, etc.) The result was a "whistle check" by the Senior Subaltern whereby the "Sr Subbie" would blow his whistle and all officers present would prove they had theirs by returning the call. The whistle call was normally performed in the Mess during Happy Hour or other relaxed, officers only, events and officers who failed the whistle check might be expected to buy the subalterns a round of drinks.
With the adoption of the CADPAT combat uniform, the lanyard ceased to be worn in The RCR.