Why are different border radiuses used for elements on same site?

Question

I've noticed that a lot of websites use border radiuses differently for different elements. I've noticed some patterns, but I'm not sure of them.

• I noticed a lot of them use completely rounded corners for search bars.
• I noticed whenever they are forms, they never use fully rounded corners.

• I noticed that when they have single call-to-action buttons, they're usually rounded.

• Twitter search bar and buttons are fully rounded, but the fields are not.

• Facebook sign up has sharp corners at the log in but not the registration.

• Gmail navigation has fully rounded corners on one side.

• Google registration has rounded corners for everything.

• Google search bar has fully rounded corners but buttons aren't.

• Spotify's form has fully rounded buttons but not fields.

• Spotify's search bar is fully rounded.

• Instagram registration has round corners.

• LinkedIn has all sharp corners.

• Imgur has a search bar with not fully rounded corners.

Is there any reason why some elements might have their corners made round, while others not?

Answer 1

I really recommend reading this article from Uxplanet: Rounded or Sharp-Corner Buttons?

You can take these points home:

-Squared Sharp designs are usually viewed as firm/serious/uniformed

-Rounded designs are viewed as friendly and harmless.

You cannot really identify a pattern from the examples you provided above. Facebook uses sharp corners for Login, and rounded ones for registration. Are they being friendly on the sign-up page to encourage you to register an account? Are you already registered and when it comes to Login it is more of a firm uniformed task, so it is sharp squared design?

Google used to have boxy sharp corners for their Search Engine, but now they use rounded ones. Why? Are they unifying their designs to be rounded for consistency purposes, or is it just a look purpose that was chosen by their designers?

I have not seen any research that would enforce using rounded corners for some parts of the application while using sharp ones for the rest. All I can think of is that Rounded ones look friendlier and Sharp ones look firmer and more uniformed, so maybe using each at different locations make sense, but how about consistency and unity?

How do we see old boxy cars designs compared to the curved rounded designs nowadays? Has Google moved from more boxy design to a curvey one? Better look, friendlier?

Answer 2

IMO - there is no general rule on this, and it depends what Designer uses what and why.
Some of the basic design principles are consistency, unity, proximity, so in cases where you create logo that is rounded, it makes sense to use also rounded buttons and input fields.
In other cases you would want to use one element with rounded corners, and other elements below with straight corners as this change captures attention of human eye.

Rounded corners are more often used as they create effect of motion, and are more known in the nature.