Maximum checkout in darts

[Explorer-8]

I see that the maximum checkout in a game of darts is 170, but I can't find out why. For example, if my first three throws are triple 20s, I get 180 points; if my second three throws are a triple 20, a triple 15 and a double 18, I get 141 points. This gives me a total of 321 points so far. I need to finish with 501 points, so 501 minus 321 = 180. So why can't my maximum checkout here be 180?

[alexanderd]

because you have to finish on a double

[bibblebub]

you have to end on a double (bullseye counts too)

[squarebear]

Because there is no double 30 on the board.

[MarkRae]

Because, under standard rules of darts, a game must finish on a double, and the bull is considered as double 25.

[Stephen_G]

If you're talking about a 9-dart finish then throwing a 180 twice still leaves you 141, so, you can either check out the way you've put in your post, or the more traditional

treble 20
treble 19
double 12.

You wouldn't leave yourself on a 170 finish unless you've mis-calculated and would throw at a single number to square it off.

Starting off throwing 180 means you'd have to score 151 to leave 170 for a 9-darter - that means treble 20, treble 19 or 17 and leaving yourself tops or double 17. Better percentage to score 360 with six and set yourself for the 141 checkout or score 354 to leave 147.

[Explorer-8]

Thanks everyone. I didn't know that it was in the rules of darts to finish on a double.