En plein (French for “in full”) is a straight-up bet on a single number in roulette, placed by putting chips squarely inside that number’s box, zero included. It is the riskiest inside bet and the best paid: a winning en plein pays 35 to 1.
On a single-zero wheel the chance of hitting is 1 in 37; on a double-zero wheel it is 1 in 38. Players often spread several en plein bets across favourite numbers or a section of the wheel, accepting frequent misses for the chance of a big payout. The phrase is also used loosely outside roulette for any bet on one single outcome. It is the simplest roulette bet to understand, but the one that loses most often.
Straight-ups in modern variants
Some newer roulette games add random multipliers to straight-up numbers and lower the standard payout to compensate. Compare the paytables when choosing roulette content for your lobby.