Life by life level by level is a slang term for a system whereby two people can play a Single player game. It works by the first player playing the game until they either complete the level, or until they lose a life. When either of these happens it then becomes the second player's turn, until they complete the level or lose a life. Wash, rinse, repeat. It gives the players an additional incentive to improve their skills; so they can keep playing for longer. Also the person who isn't playing at the current time can watch the current player's technique to help them if they have to face that level.
The phrase is meant to be said quickly hence why it isn't written life by life, level by level.
- Person 1: "What are you playing?"
- Person 2: "Sonic the Hedgehog."
- Person 1: "I want to play!"
- Person 2: "Life by life level by level?"
- Person 1: "Yeah."
- Person 2: "OK."
Some games have a modified life by life level by level system already built in. Super Mario Bros., Super Mario Bros. 2, Super Mario Bros. 3 and Super Mario World all have a life by life level by level system built in as a 2 player mode. The only difference is that each player's progress is independent of the other player's. So if Player 1 completes the 1st level it's now Player 2's turn, but they also start on the first level and not on level 2.