I first learnt about this concept in the book Game Design Workshop: A playcentric approach to create innovative games (2nd ed) where the four areas are nicely covered. I'm surprised this concept was not more known and I could only find when it was first made public in Eric Todd's presentation Spore: Preproduction through prototyping (ppt) at 2006 GDC.
Follow me while introducing this great tool. To start, let's ask: What do I need to prototype?
Digital prototypes are made using only the elements needed to make them functional. They are not finished games, and if you spend too much time making them like finished games, you will defeat the purpose of prototyping at all.
Prototypes answer questions
The goal for creating a prototype should always be answering questions on parts of the design that need clarity.
Let's say that I want to know if the story of my game is going to be interesting enough. You can create a short fiction story and see if others are geting excited and engaged with it. The same can be applied to different areas of a videogame. But because of how complex a game can be it's difficult to test things in isolation.
Figure out what are your questions about the design. Can some of them be solved with a prototype? multiple prototypes?
Areas of investigation
There are four main areas in which we can fit that question that we need to solve:
Game mechanics: The formal aspect of the game. Winning, losing, economies, etc. It sometimes involves mathematics.
Kinesthetics: The game feel. Idea is that the interface is an extension of your body into the game world. How does the whole interaction loop feels? According to Steve Wink's Game Feel components are: Input, response, context, polish, metaphor and rules.
Aesthetics: Mostly talking about visual and aural aesthetic. That's visual art, music and sounds.
Technology: During preproduction you’re looking for ways to reduce risk and if you’re doing something new, some of that risk may be technical. Prototyping technology is about getting answers to successfully implement gameplay concepts, adapting to new platforms or defining pipelines.
The Prototyping X helps limit the scope of prototypes
The main rule of the Prototyping X is to avoid researching or prototyping two areas that pull you in opposite directions at the same time.
The reasoning behind this is that these areas involve conflicting goals:
Mechanics vs. Art:
Mechanics are about making everything work internally, almost at a mathematical level. It involves making lots of adjustments, changes, and tests. If you have to worry about keeping everything pretty with every change, you’re going to run into problems. On the other hand, to test art styles it’s often enough to create concept art or mockups. Mechanics can be ugly and can be tested in spreadsheets or with an improvised tabletop game made from scraps of paper.
Technology vs. Game Feel:
Game feel is about everything being fluid—it's the feedback loop between the player and the game, and how they communicate. If you’re trying out a new technology, the last thing it will be is polished and refined. Testing a new technology doesn't mix well with game juice and usability.
Depending on the question your are trying to answer with your prototype you will focus on 1 or 2 areas on each prototype.
Some examples:
Mechanics + Technology: Procedural level generator; Inusual game controllers.
Mechanics + Game Feel: Playable game feature with placeholder art; Testing different metaphors or themes.
Technology + Art: Technology demo; Shaders.
Art + Game Feel: Fake trailers and screenshots; Playable with single interactions.
If you find yourself wanting to test something in more than 2 areas at the same time, consider if you are actually just trying to start making the game instead of a prototype.
As you get more questions answered and get into the iterative process you will start to work in more areas at the same time. By then, your focus should be about integrating all the moving parts.
Consider another resources
Before going directly for a digital prototype consider all the other resources and options available to put your question to the test: Physical objects, toys, pen and paper, people, people following orders, modded games, roleplaying. The list has no end. What you need is to recreate only the bits of the experience that give you the information you need.
Closing thoughts
It's 2025 at the time I'm revisting this article from 2013 and I still haven't found many references to this concept. I have always shared this with excitement and it's been very difficult to find more real world examples of this tool in use (other than my own). If you know more I'd love to know about them. Look for my contact info at the bottom of the page.