To illuminate a screen by displaying all odd lines in the frame first and then all even lines. Interlacing uses half frames per second (fields per second) rather than full frames per second.

The interlace method was developed for TV broadcasting because the allotted bandwidth for TV channels, defined more than a half century ago, was not sufficient to transmit 60 full frames per second. Interlacing with 60 half frames was visually better for moving images than 30 non-interlaced full frames.

Interlace Vs. Progressive Scan
Interlaced screens display every other line (1-3-5 etc., then 2-4-6, etc.), while non-interlaced screens, known as "progressive scan," display lines consecutively (1-2-3 etc.).

All non-digital TVs are interlaced. Older CRT computer monitors were also interlaced at their highest resolution and progressive scan at lower resolutions. Some digital TV standards are interlaced, such as the high-definition 1080i format, and HDTV sets support both interlaced and progressive scan signals (see HDTV).

Modern CRT and flat panel computer monitors are all progressive scan. See deinterlace, vertical scan frequency and interlaced GIF.