In game development, “vertical slice” is one of those terms that you might hear without really thinking about what it means and what it’s for.
Different companies and different devs might have slightly different understandings of exactly what a vertical slice is, but what we have here should be enough to give you a good idea of what a vertical slice is, how it can help you through pre-production, and how it can guide the development of your indie game.

What is a vertical slice?
A vertical slice is a key milestone in the development of your indie game. For most studios and developers, this is a very early stage of the development process that will usually be put together once developers have finished prototyping the core concepts and ideas they want in their game.
The vertical slice is usually put together during the pre-production stage and is a useful way to show investors how the game will play and it can even be handed out to playtesters for feedback on the key features.
A vertical slice should include all the approved design concepts, but not necessarily design concepts in their final form. Most of the features included in the vertical slice will still need to be changed or polished before they’ll be good enough for the final product.
New design concepts can also make their way into the game after the vertical slice has been made, but keep in mind that the majority of features should feature in a vertical slice and too many later additions could turn into feature creep.

What does a vertical slice do and why is it important?
A vertical slice is supposed to show off the intended player experience and act as a proof of concept for the whole game. It will give an overall impression of the developer’s intent. Even though it won’t and shouldn’t be polished, the game’s general player experience and flow should be clear.
You could think of the vertical slice as a storyboard for the experiential part of your game. It’s the most basic way you can represent the game and see what works, what doesn’t, and what needs to be retooled, fixed, or tightened.
Now it’s time to see if your vertical slice is any good.

What to do if your vertical slice isn’t any good
In the event that your vertical slice isn’t cohesive, fun, or doesn’t achieve the objectives you set for your game, you should look closely at what’s going wrong and where the issues are.
The worst-case scenario is that your vertical slice is not worth pursuing any further. That said, putting together this vertical slice will save you a lot more time than making an entire game before finding out that it’s no good.

What to do if your vertical slice is good
Great! A good vertical slice shows that the underlying concepts that make up your game are fun or interesting and means that the idea for the game is worth pursuing.
It also means that the features in your game work as you intended and you can move on to your next production milestones without having to go back to the drawing board.
The development milestones after your vertical slice will either be set by you or your studio, but Alpha is usually the next stage of development and this means that your game has moved from pre-production to production.
From there, you can keep testing and iterating on the ideas present in your vertical slice. Don’t forget that your vertical slice is the most powerful tool against feature creep and for indie devs with limited manpower, time, and budgets, it can help you in the fight against feature creep!