If you are playing roulette you will soon be introduced to the Martingale system and/or its many variations. Beware. In theory it is an unbeatable system. Martingale systems are based on a theory of doubling the stake plus a bit more every time you lose on an even-money chance. Be aware that should you bet just eight times without success your cumulative loss will already be 255 times your original stake.
It was devised by one Jon Harris, apparently ‘an ardent student of gambling’. The system is designed to cover 30 of the possible 37 numbers and make a ten-unit profit on each winning spin. The system is played on one of the dozens and either high (19 to 36) or low (1 to 18). The only numbers against you if you go high are zero and 13 to 18; or zero and 19 to 24 if you go low.
If chances simples wins you lose 20 on the dozen but win 30. You might opt to wait until the numbers against you have come up twice in six consecutive spins. You might not. Always play on a wheel with a single zero if possible, but watch out for casinos usually American with a rule that zero coming up also means that all even money bets are lost.