It's the PDC tournament that causes confusion every year. The World Grand Prix is underway in Leicester, with many viewers left asking why players aren't beginning legs by aiming for treble 20.
Indeed, the Grand Prix event is considered unique on the circuit, for enforcing one rule that no other tournament on the tour uses.
Usually, professional players are only required to hit one double during the leg - when closing out a win.
However, in this tournament entrants must both start and finish on a double (or bullseye). It's a rule that can prove a frustrating one for those involved, with players not able to score a single point until a double has been notched.
The regulation was during the opening night. He was taking on the iconic Raymond van Barneveld and looked in control, taking the first set in a first-to-two format.
But it was in the second set where he suffered a momentarily lapse, beginning a leg by nailing treble 20. To his credit, the 29-year-old was able to laugh when he realised his error, and recovered to take the match in the deciding set.
The World Grand Prix is the only double-start tournament in the PDC calendar. It's a notion that has long divided onion, both with supporters and the 32 who make up the field.
But despite the strange format, the tournament has still produced three nine-dart legs since the inaugural version in 1998. The first came from Brendan Doolan in 2011, before a famous clash between James Wade and Robert Thornton three years later.
Both players hit perfect legs in the same match, the first time this had happened in any televised darts event. On all three occasions, the leg began with a score of 160 (starting on double 20), followed by 180, followed by finishing 161 with treble 20, treble 17, and bullseye.
Monday's opening night has seen the likes of Jonny Clayton, Nathan Aspinall, and all win though to the second-round. But one of the most dramatic clashes involved 17-year-old on his tournament debut.
Littler lost the first set 3-2 to Rob Cross, before levelling the match by taking the second by the same scoreline. But it was former world champion Cross who finished stronger to take the decider 3-1.
