Legacy broadcast standard: alternating odd/even scan lines per frame — flickers on modern monitors. Obsolete now; only matters for archival material and broadcast compliance.
In interlacing, a video image is divided into two fields—first, all odd scanlines are transmitted, then the even ones. This sounds technically elegant but was necessary for television sets from the 1950s to the 2000s to achieve an acceptable flicker frequency despite lower bandwidth. The old PAL standard (50i for Europe) or NTSC (60i for the USA) work this way: two fields of 25 or 30 Hz optically result in 50 or 60 Hz image changes—enough for the eye not to consciously perceive the flicker. This is the crucial point: interlacing was a compromise between bandwidth and perception.
In modern workflows, interlacing is a nightmare. Cameras have long been shooting progressively—that is, image by image, line by line from top to bottom (1080p, 4K DCI, etc.). If you digitize legacy material or work with older broadcast equipment, you have to separate or combine the fields again. Moving objects show combing artifacts because the upper and lower fields are temporally offset—one line comes from frame 1, the next from frame 1.02. When zooming in or post-processing, this quickly becomes visible and ugly. Professional deinterlacers (software or hardware) try to reconstruct the missing lines, but you don't get true image quality back—only an approximation.
The topic remains practically relevant only for archival material: old video tapes, broadcast archives, scanned film digitizations with interlaced artifacts. If you have to work with it, deinterlace before color grading and scaling—not afterward. Most NLE systems (Premiere, Final Cut, DaVinci) have integrated deinterlace filters; in DaVinci, a dedicated module in the Color tab is responsible for this. For analytical work on the material, it helps to view the fields individually—then you immediately see where the temporal offset causes problems.
In short: interlacing is dead, but it still lurks in archives and old project files. Recognize it early during ingest, choose the right deinterlace method, and convert to progressive formats—then your material will be future-proof.