Internal (inside) and external (outside) bets usually give a result of the lowest odds, generally 1:1 or 2:1. The sections are marked in the following way: the 1st twelve, the 2nd twelve, the 3rd twelve, red and black, even and odd, 1-18 and 19-36. You may place your bet on any of these sections. The principle – in a few words – is that when you place your bet you hope that the ball will fall into one of those sections. As there is a good chance that the ball will fall into one of those numbers, the odds will be 2:1 which means one in three chances. If it’s a one-in-two chance that the ball will fall into the chosen section (for example black or red), then the odds will be 1:1.
Besides these external bets there are also so called internal ones. These internal bets are bets that are made inside the betting grid, including the zero or zeros (in case it’s an American variant of Roulette, remember?). By placing a single bet on one number you reduce the winning chances. But in case you win you will get odds of 35:1 because the theoretical frequency to get this very number is rather low.