The Tiers du Cylindre bet in roulette covers 12 numbers that are located on the third of the wheel opposite the "Voisins du Zero" section. These numbers range from 27 to 33 and include a mix of straight-up, split, and corner bets. The bet is designed to cover a portion of the wheel, providing players with the chance to win on the numbers in that specific section. It is a popular choice in French and European roulette, offering a strategic way to bet on a specific area of the wheel.