I was in Iceland this past week - beautiful country; I recommend checking it out.
While in Reykjavik (the capital of Iceland), I stayed at a hotel which had a major design flaw - the front door of the building had a large handle on it, but was a "push" doorway. I was traveling with a group of doctors, all of which are very intelligent people, and yet still there were multiple attempts to pull the push door. It was right out of a farside comic – one of the doctors even gave up on the door altogether, assuming it was locked.
What the Reykjavik hotel door was missing was constraints; by giving people a handle, the door encouraged pulling. If the door handle was removed, people would be constrained by the only available option, to push, resulting in a substantially higher usage success rate.
Constraints are an essential element of design. For example, the Toronto subway system has newspaper recycling boxes with large circular openings on their top, allowing for objects much larger (and a different shape) than newspapers to be placed in them. As a result of this lack of constraints, objects other than newspapers are incorrectly placed in the box. Similarly, many roads are missing the constraint of side rails/dividers - without those side rails, the chances of head-on collisions increases dramatically.
Constraints aren't only for the physical world; they can also be implemented online to ensure users take a desired action. For example, users can be constrained in an e-commerce site's checkout process by removing navigation links which lead to pages outside of the shopping cart. Similar to how a side rail constrains cars, preventing them from driving on the wrong side of the road, removing site navigation links constraints users, making it less likely they will leak out of your conversion funnel.
Constraints can also be used for conceptual designs, such as the creation of a brand. While in Iceland, I learned that the Vikings never developed writing; instead they had a strong oral tradition. As it is hard to remember long stories, the Vikings used mental constraints to simplify the task - poetry. A Poem is an execution of constraints; the use of rhyming and syllables limits the possible selection of words in a sentence from thousands to dozens. When meaning is considered, the potential selection of words is limited further. Hence, constraints can be used to help remember things.
Applying the concept of mental constraints to branding and online marketing, you can increase the ease-of-recall of your brand by utilizing mental constraints. Youtube is a great example of this; when attempting to recall the brand, a person only has to remember one word – you. Once the person recalls the word “you”, they can then construct the remainder of the name mentally – a very limited number of words rhymes with “you”, of which an even smaller number has meaning related to tv / videos. (Although "tube" doesn't rhyme with "you", "two" does, and "tube" is similar enough to "two" for the connection to be drawn almost automatically)
Similarly, when choosing a domain name or site slogan, mental constraints can be very helpful. One word domain names are very expensive to acquire, and two word domains can be hard to remember. Rhyming domain names are much easier to recall, but coming up with the second word can still be problematic; an ideal name would be both rhyming as well as explanatory of the site's contents, thereby further constraining the possible name variations.