Corners in cribbage
Corners in cribbage is a term which derives from the layout of the cribbage board. A traditional board is laid out in two rows of 30 holes for each players, and the winner is the first to 121 points - or twice around the board.
Consequently, when you reach the end of a row and start pegging up the next row, you have turned the "corner". Sometimes wagers or bonuses are placed on corners, won by the first player to reach that corner, although there is no additional score for corners in the standard cribbage rules. Some boards have extra markers for corners scored by each player.
The rows in between the corners are often called "streets", so a player with between 1 and 30 points is said to be "on First Street", and the home straight between 91 and 120 points is known as "Fourth Street'. The streets and corners are especially important in understanding positional strategy in cribbage.
(Thanks to Sam Van Wyck for this info.)