The regulator necklace is a popular method to manage your
alternate air source. The necklace concept derives from technical divers
that had a need to keep their back-up regulator close to their mouth
for instant availability. If you wish, you can make your own regulator
necklace instead of buying one; there are several methods. A easy method for most divers is to tie a knot in each end of the desired
length of 1/8 in or 5/32 in diameter bungee cord,
then burn the ends of the cord to keep them from unraveling. Cut and
remove the cable-tie holding your second stage mouthpiece (take care not
to cut your mouthpiece), lay the ends of the bungee cord along both
sides of the mouthpiece and then secure with a replacement cable-tie and
trim. Another method that allows the necklace to be added without disturbing the cable-tie is to tie two fisherman's knots in the bungee to make an adjustable opening that is snugged onto the regulator mouthpiece, see this YouTube video. There is no 'standard' length, but avoid the tendency to make it too long; a starting point for determining the finished length of the loop is your neck size plus about {6 to 8 in | 15 to 20 cm}.
When donning your gear, the necklace holding the backup regulator is
placed over your head first, allowing the second stage to hang securely
under your chin. Then put the primary regulator in your mouth. It may
take one or two dives to get comfortable with the new regulator
position, after that you won't even be aware it's there unless you need
it.
