An aspect ratio can be defined as the ratio of the height to the width of a rectangular display. Since smartphones have a rectangular form factor, aspect ratio on smartphones can be defined as how tall the display is.
Aspect ratio is gotten by dividing the height of a screen by the width. Aspect ratio is usually written in a ratio format with height first and width second i.e. height:width.
So if a screen was 12cm tall (height) and 9cm wide (width), the aspect ratio can be gotten by dividing both numbers with a common denominator. This is done until they are no longer divisible by the same common denominator.
12/9 (divide both numbers by 3) = 4/3
There is not a common denominator that can divide both 4 and 3.
This means that a screen that is 12cm in height and 9cm in length has an aspect ratio of 4 by 3 or 4:3.
In the early days of smartphones, when a lot of people were still making the leap from devices with buttons to the touch screens of iPhones and Androids, especially Androids. Smartphones had huge bezels and those navigation buttons that helped you find your around the phone. Then most phones had a 4:3 display.
Read: Smartphone displays
Samsung Galaxy Mini (2011)
The smartphone era did take off and smartphones went into full gear. As more content was made for smartphones, consumers started demanding for larger displays. This was in order to enjoy content better and the smartphone companies obliged.
Screens began to get longer and taller and that is the norm today. Nowadays though, taller screens seem to be more of an aesthetic or fashion statement than anything else.
Types of Aspect ratios
The major aspect ratio in use on smartphones right now are: 16:9, 18:9, 19:9 and 19.5:9.
That being said, 20:9 is fast becoming the norm these days as screens are growing taller and taller.
Modern smartphone companies are trying really hard to eliminate bezels. This is so that the user can have the entire face of the smartphone for their personal viewing consumption.
An iPhone 3GS compared with a modern Redmi Note 9 Pro. Notice the minimal bezels on the more modern device?

The craze for taller displays has led to a lot of innovations designed to create more screen space. These include the notch (wide, infinity, V and teardrop), pop-up and slide-up cameras as well as under display cameras. All of these are designed to provide smartphone users with more screen real estate and very little obstructions.
It is important to note that some smartphone users are calling for a return back to the 4:3 days when smartphones fit snugly into hands. That ship seems to have sailed. This new tall bezel-less era has come to stay.
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Thank you. Informative.
What are bezels, please?
Bezels are the dark colored parts of screens on old Smartphones.
These days they’re can no longer be found as manufacturers are giving more and more real screen estate to the users.
Thank you!!