We all watch movies serials and videos on television, youtube and on other devices, but we never notice the black bars on the top and bottom of the screen because we are that habitual of watching our favourite videos with the bars that they become blind to our and mind never focuses on these black side bars.
The reason behind these bars is that the original movie or video in a wide aspect ratio for playing in movie theatre and movie theatres and other common devices like phones, laptops have rectangular screen but the television has more of a square shape, so to convert square tv screen into rectangular screen, these black side bars are used in movies and videos.
Original Video Shot For Large wide screen.
To fit the video into tv Aspect Ratio
To fit the large aspect video into a small screen, they first used a compression technique. In this technique, video is compressed to such aspect ratio that it covers all the screen sizes but this technique had a drawback that video may look stretched tall-like on screen.That’s because they compressed or squished the rectangle to fit your square.
To Solve this LETTERBOX technique was used cause people want to see the full movie . In This method both WIDTH and HEIGHT is shrunk to fit your dinky Square of a TV, When the image was shrunk to fit the TV Screen, they were left with a gap at the top and bottom of the screen. So they simply fill it in with “Black” to avoid the snowy screen.
To fit the video into tv Aspect Ratio both width and height compressed .
But now a days videos are shot in a proper HD aspect ratio and theres no need to compress the videos as most of the HDTVs has the HD ASPECT ratio, but many youtubers and other video producers uses this black borders on the top and bottom of the screen to give it a more cinematic look.