Rainer
Contributor
And neither is an OW class...
Really?? I was taught to use a pony bottle in my NASDS AOW course in 1986 as my instructor prepared us for solo diving. More recently I have seen pony bottles introduced in PADI AOW for deep diving.
Doubles and redundancy are obviously introduced early in NSS-CDS and NACD courses though these are not likely considered mainstream agencies.