An interlaced GIF image is displayed in 4 passes.
GIF interlacing changes the order in which horizontal raster lines are presented so that a recognizable version of the whole image can appear before all image data has arrived. The method was especially useful when images were downloaded over slow connections, because viewers could see a rough preview instead of waiting for the image to fill from top to bottom.
The first pass displays line 0 from each eight-line strip. The second displays line 4, the third displays lines 2 and 6, and the fourth fills in lines 1, 3, 5, and 7. Together, these four passes reconstruct the complete image in its normal row order.
The number 4 describes the interlacing schedule, not the number of colors, animation frames, or compression stages. Non-interlaced GIFs do not use this four-pass arrangement; their rows are stored sequentially. GIF interlacing is also different from PNG’s Adam7 method, which uses a seven-pass two-dimensional pattern.