11. Mini-games.

A small game within a big game, usually optional, sometimes not. Not the same as multi-level gameplay; a mini-game feels very different from its parent. WarioWare consists of nothing but mini-games. Mini-games often destroy the player’s immersion, but offer a different set of challenges from those in the overall game. Sometimes the mini-game is actually better than the overall game. First use: unknown.

12. Multiple difficulty levels.

Designer John Harris has observed that older games, especially coin-ops, were intended to measure the player’s skill, while the newer approach is to provide the player with an experience regardless of his skill level. The old-fashioned school of thought is that the player is the designer’s opponent; the new school is that the player is your audience. By offering multiple difficulty levels, we make games available to larger audiences, which also includes handicapped players. First use: unknown.

13. Reversible time.

Saving and reloading is one thing, but sometimes what you really want is what as kids we used to call a "do-over"-a chance to correct an error without the hassle of a reload or going back a long way in the game world. Best-known example: Prince of Persia: The Sands of Time, 2003. When you made a mistake, you could reverse time for ten seconds. To prevent you from using it continually, each usage costs you a certain amount of sand, which has to be replenished by defeating enemies. The game also let the player see into the future to help with upcoming puzzles, another clever innovation. Possible first use: Blinx: The Time Sweeper, 2002, in which collecting up crystals in various combinations gives the player a variety of one-shot time control commands.

14. Coupled avatars.

In this slightly oddball innovation, you play an action or action-adventure game using two quite different avatars with complementary abilities. Sometimes they work together as one; at other times you have to choose which to use, or are required to use one or the other. Not the same as two separate avatars like Sonic and Tails. Possible first use: Banjo-Kazooie, 1998.

15. Sandbox modes.

The term refers to a mode of play in which you can fool around in a game’s world without being required to meet a particular objective. By far the best-known sandbox modes are in the later Grand Theft Auto games, contributing greatly to their popularity. Sandbox mode is normally used to describe special modes within otherwise goal-oriented games, not open-ended games like SimCity. Sandbox modes also sometimes afford emergent behavior, events arising in a game’s world that were not planned or predicted by the designer. First use: unknown.

16. Physics puzzles.

Many real-world games involve physics, but they’re usually tests of skill. The computer lets us create physics puzzles, in which you try to figure out how to accomplish a task using the physical properties of simulated objects. They’re about brainpower, not hand-eye coordination. Possible first use: The Incredible Machine, 1992.

17. Interactive drama.

There’s only one of these, but someday its descendants will change the world. Façade is a first-person 3D game released in 2005. In Façade you play the friend of a couple whose marriage is in trouble. You visit their apartment for an evening and converse with them by typing real English sentences; they respond with recorded audio. Depending on what you say, you can influence their relationship—get them to reconcile, cause one or the other to leave, or even anger them so much that they throw you out. It’s role-playing in the real meaning of the term: no stats, no combat, no treasure, just dramatic interactions—with a couple’s future happiness at stake. Many designers consider the “holonovels” from Star Trek: The Next Generation to be the holy grail of interactive storytelling; Façade is an important advance on the quest.

Next: Input innovations