The aspect ratio of traditional PAL and NTSC television pictures, originally chosen to match the 35mm film format of the time. All broadcast television pictures were 4:3 until the introduction of high definition when a wider image was considered to be more absorbing for viewers. For display tube manufacturers the most efficient aspect ratio would be 1:1 (square) as this is inherently the strongest shape, uses less glass and weighs less.
More glass is used in 16:9 tubes so they are more expensive to produce. Such restraints do not apply to today’s TV screens and monitors using LED and Plasma technology.