There are a few different ways to play it. In most cases I think folks just set the 16 pill aside.
But for those who do not, the most common use for the 16 pill is that it represents the cue ball. For example here is one set of Kelly Pool rules using the 16 pill:
Kelly Pool – 2 to 15 player game of elimination – pocket your secret ball
- Kelly Pool rack: triangle (1-15) – 1 at the foot spot, 2 and 3 on the wings, 15 in the center
- Player order determined by draw from pill bottle
- Secret ball assignments determined by separate draw from pill bottle
- Must hit lowest ball first (as in 9-ball); called shots not necessary
- Spot sunken balls on fouls
- Pocket your secret ball to win
- When your secret ball is pocketed by another player, you are “dead”, but you keep playing in rotation
- You must tell other players you are dead before your next shot – $1 penalty if you don’t.
- Unless 15 is your secret ball, combos on 15 ball not allowed (must spot if so)
- If no one sinks their secret ball, person who sinks 15 ball (last ball on table) wins
- If you draw the 15 pill, you must sink the 15 to win (and for you, combo on 15 is allowed)
- If you draw the 16 pill, you are assigned the cue ball. You must hit the lowest ball first, then scratch the cue ball to win. If someone else hits the lowest ball first then scratches, you’re dead.
Here is another mention from AZB of the 16th tally ball being assigned to the cue ball:
I just read that the 16 could be assigned to the cue ball. If you have the 16, you just have to sink the cue ball to win, but only after hitting the lowest ball on the table first. If you miss, and others realize you have the 16 pill, they'll scratch for you, after low-ball contact, in which case you're out.
Another account from Ontario, Canada recalls a similar usage for the 16th pill in Kelly Pool:
When we played this in the 1950s I believe the 16 was 'wild' and that it was impossible to 'kill' the holder.
And another, also from Canada, says they never used the 16th pea:
I remember playing this all the time, on a 6' X 12' pool table in the 1960s but I don't believe there was a #16 pea.
Also, there are a lot of other games which use a tally ball set, and use more than 15 pills. According to the Chicago Billiard Museum:
It appears that since the peas were not just used for Kelly Pool. There were sets of bottles and peas prior to the 1890s with numbers into the twenties and possibly even higher.
"Pin Pool" would be a good example of a game played much prior to 1890, that involved pea sets that may or may not have included only 1-15.
One such game is called "Crazy 8":
Crazy 8 is played using pills 1 through 16. The 8 and the 16 are "freebees" and up to four players can play. A player can shoot in rotation from the 1 up, from the 7 down, the 15 down, or the 9 up.
No one knows what the other players have. When it is your turn, you have to declare when you are shooting the 8 ball to win. If you make the 8 ball to win, you must show what 2 pills you had, that are now gone from the table.
A complete set of pills or tally balls has the 8 pill and the 16 pill. If you draw the 8 pill or the 16 pill it is free and you only have to make the remaining pill / ball to be on the 8.
There is a lot more strategy and skill than there seems. Of course it is meant for medium skilled players as a good player can just run-out at will. We had a ball playing this in the late 1960's. You had to watch out for players with more than 2 pills in their pockets though.