A Boston Marriage Play
When you check in first position, you are not giving yourself a free card; you are offering your opponent a free card. That player can decide whether to take it or to bet. Consequently, in first position you have to bet some hands you wouldn’t bet in last position because you do not want to give your opponent the option of checking for a free card with the worst hand.
When a hand is reduced to two opponents, the player who acts first cannot give himself a free card, but the player who acts second can. If you are second to act and your opponent has checked to you, you should bet when you are pretty sure you have the best hand; but if you suspect you have the worst hand, you can check and give yourself a free card.
With a marginal hand you should bet in first position, especially if you don’t fear a raise, because if your opponent has a worse hand than yours, he will check behind you when you check, making you wish you had bet. On the other hand, if your opponent’s hand is indeed better than yours, he will bet when you check. So you couldn’t give yourself a free card anyway.