YouTube originally offered videos at only one quality level, displayed at a resolution of 320x240 pixels using the Sorenson Spark codec (a variant of H.263), with mono MP3 audio.
In June 2007, YouTube added an option to watch videos in 3GP format on mobile phones. In March 2008, a high quality mode was added, which increased the resolution to 480x360 pixels.
In November 2008, 720p HD support was added. At the time of the 720p launch, the YouTube player was changed from a 4:3 aspect ratio to a widescreen 16:9. With this new feature, YouTube began a switchover to H.264/MPEG-4 AVC as its default video compression format.
In November 2009, 1080p HD support was added. In July 2010, YouTube announced that it had launched a range of videos in 4K format, which allows a resolution of up to 4096x3072 pixels. However, it was lowered to 2048 x 1536 as of 2012.