What does form follows function really mean?

Louis Sullivan, an American architect of the 19th century, coined the famous phrase "form follows function." This statement is so common among product designers today that we typically take it for granted. But this quote can help us become better at design.

Here are two critical things we should remember about 'form follow function':

Point A: Design constraints define how a product should look

Let's assume you asked to design a new vehicle and have no information about how this vehicle will be used. For a moment, you might think that you've got a lucky ticket—since you have total creative freedom, you can design whatever you want. But as soon as you start designing, you realize that this freedom can easily lead to a nightmare because when you don't understand how a product you create will be used, it's extremely hard to design something useful.

Constraints guide the design process. The more specific constraints you have, the easier it is to design a decent product. More constraints often lead to a simpler design.

If you know how a product will be used, you can use this information in the design. The more nuances you know, the better your solution will match the goal. For example, when you know that small shops use a vehicle to deliver cardo in busy areas of a large city, you are likely to make it look like a van and focus on its ability to handle large cargo without being too large.

That's why defining success criteria (what a successful design will look like) should be the first thing you do in the design process. It might sound a bit odd. How should you know what a product should look like even when you haven't started designing yet? The point is—you need to learn more about the problem you're trying to solve. It's clear that it's impossible to define success criteria when you don't understand the problem you are solving. You have to invest in design research to understand what you aim to achieve.

When you have success criteria, every decision you make serves a specific goal. Without clear criteria for designing a product, you end up moving into the dark, constantly trying to experiment.

Designing backward (when you first determine what the final outcome should be and then plan how to go there) is a very popular technique among product designers. The ultimate goal you define early in the design process becomes your North Star which guides your entire design process.  

Point B: People should understand how to use a product just by looking at it

Designers use the form to communicate a specific message to their users. Object's form can tell people how they can use it. Most of the time, this understanding is driven by user intuition and knowledge. Visual attributes of an object give users specific ideas about the object. For example, a shape can be used to communicate the function. If the object has a rectangular shape with a popping element that invites users to press it, users will think it's a button.

The shape of your design dictates what your product does. If it looks like a button, it should act as a button.  

It's possible to use not only the shape but color to imply the function. For example, we can use a contrasting red color for potentially dangerous actions to warn users. The red color in nature means danger, so the users follow their intuition and be careful with things colored in red. This is done subconsciously—users create associations by looking at a product.

The ability to understand how things work is integral for product success. Users shouldn't have to read manuals to learn how to use a product; instead, they should be able to understand it just by looking at a product.