121 checkout
121 checks out in three darts under double out: T20, T15, D8, for 60 + 45 + 16. If the treble 20 lands in the single 20, 101 is left and still on: T17, BULL. Should the second dart, at the treble 15, drop into the single 15, 46 is left, out of reach with the last dart: S14 leaves 32, so D16 on the next visit. With only two darts in hand, T20, T15 leave 16, so D8. Under straight out, where any segment can finish: T20, T17, S10.
When a dart strays
A missed treble or double often drops into the single of the same segment, and a missed bull into the 25 around it. Here is what that leaves, and what comes next.
With fewer darts in hand
The finish is then out of reach: these darts set up the next visit.
Under the other checkout rules
Neighbouring scores
Let DartSync work out your finish
In a game, the scoreboard shows the route every visit, following the checkout rule you chose.
Play a game of 501