History Lesson: What Is a .gif File? CompuServe, the LZW Patent, and GIF89a

What is a .gif file?

A .gif file is an image stored in the Graphics Interchange Format. Spell the idea plainly: each pixel is usually an index into a color table (palette) of up to 256 colors, and those indexes are packed with LZW (Lempel-Ziv-Welch) lossless compression — the encoder rebuilds exact palette indexes, not a blurry approximation. The file opens with six ASCII bytes, either GIF87a or GIF89a, so readers know which revision they are holding.

CompuServe introduced GIF on 15 June 1987 for its online service’s file areas, replacing an earlier black-and-white run-length scheme. Color weather maps, icons, and screenshots needed something denser than raw bitmaps on 300–2400 baud modems. Because the specification was published openly, GIF spread beyond CompuServe onto bulletin-board systems, early web pages, and countless desktop editors. The everyday search “what is a .gif file” is usually someone staring at a looping meme, a transparent logo, or a 1990s website asset that never became a PNG.

What is the difference between GIF87a and GIF89a?

GIF87a is the original 1987 stream: logical screen, color tables, image descriptors, LZW image data, optional interlacing, and an end-of-file trailer. Multiple images in one file were already possible, but timing, transparency, and vendor hooks were thin.

GIF89a — CompuServe’s programming reference dated 31 July 1990, widely mirrored (including the W3C text copy) — keeps that core and adds extension blocks. The Graphics Control Extension (GCE) carries a transparency flag and transparent color index, a disposal method (what to do with the prior frame), a user-input flag, and a delay time in hundredths of a second. A Comment Extension holds human-readable notes. A Plain Text Extension (rarely used today) could draw grid text. An Application Extension lets vendors stash application-specific data — the door Netscape later walked through for looping.

How the bytes work: signature, palette, LZW, interlacing

After the six-byte signature comes a Logical Screen Descriptor: canvas width and height, packed flags for a Global Color Table, color resolution, and a background color index. If the flag is set, a Global Color Table follows — up to 256 RGB triplets the whole file can share. Individual frames may instead (or also) carry a Local Color Table. Pixel data is not raw RGB; it is a stream of palette indexes compressed with LZW, the dictionary coder Terry Welch described that Unisys patented as U.S. Patent 4,558,302.

Interlacing is the progressive-display trick that helped dial-up life: scan lines are stored in a four-pass order so a coarse recognizable picture appears before the whole file arrives — useful when aborting a slow download mattered. GIF89a’s GCE then layers timing and one-bit transparency on top. None of that invents true 24-bit color; the ceiling remains the palette size.

CompuServe, Wilhite, and why GIF spread

Steve Wilhite led the work at CompuServe in Columbus, Ohio. Contemporary profiles and his later interviews describe a service that needed compact color graphics for downloads; he recalled wanting the format in his head and programming toward it, with an early test image of an airplane. CompuServe published the specification rather than locking it inside one client, which is why BBSs, Mac and PC paint programs, and then web browsers all grew GIF readers.

LZW beat simpler run-length encoding on many graphics of the era, so a color screen dump that would have choked a modem as a raw bitmap became shareable. Interlacing and a small signature made the format friendly to imperfect networks. By the mid-1990s GIF sat beside JPEG as one of the two pictures the early web actually shipped — JPEG for photos, GIF for logos, bullets, and eventually loops.

The Unisys LZW patent, “Burn All GIFs,” and PNG

CompuServe chose LZW believing the pieces it needed were free to use. Unisys held the patent and, after private talks beginning around 1993, reached a licensing deal with CompuServe announced in late 1994. Developer communities learned that shipping GIF encoders could mean royalties. Press coverage from the period records CompuServe’s surprise and Unisys’s fee demands; exact dollar figures in secondary retellings vary, so this lesson does not invent a price list.

The backlash was cultural as well as technical. Activists organized “Burn All GIFs” protest days. Engineers designed PNG (Portable Network Graphics) as a patent-unencumbered successor for still images — better compression options, truecolor and alpha, but no GIF-style multi-frame animation in the core PNG format. Browser support for PNG took years to catch up; GIF kept its meme and banner niche anyway. Unisys later stated that its U.S. LZW patent expired on 20 June 2003, with counterpart patents in several other countries expiring in 2004 (commonly cited dates include 18 June 2004 for some European counterparts). After those expirations the licensing fight left the format; the files on disk did not.

Animated GIFs and the Netscape looping extension

GIF89a can already store several images with per-frame delays via the Graphics Control Extension. What the 1990 spec did not standardize was “play this forever.” Netscape filled that gap with an Application Extension identified as NETSCAPE2.0: a small block that sets a loop count (0 meaning loop forever in the usual reading). Support appeared with Netscape Navigator 2.0 in the mid-1990s and spread to other browsers. That unofficial-but-everywhere extension is why a silent looping clip became the default meme container long before short video tags were common.

Important precision for converters and archivists: JPG and ordinary single-page PNG / PDF outputs are still-image targets. Re-encoding a multi-frame GIF into those formats does not magically preserve a timeline — you keep picture data suited to a still, not a browser loop. This article does not claim frame-extraction tools or animated output; it only notes what the containers are for.

Soft g or hard g? The pronunciation dispute

CompuServe’s own documentation joked that choosy programmers choose “jif,” echoing a peanut-butter slogan. Steve Wilhite kept defending the soft g. At the 2013 Webby Awards he accepted a Lifetime Achievement honor with a five-word slide: it is pronounced “jif,” not “gif.” Hard-g speakers answered that Graphics starts with a hard g. Dictionaries often list both. The format does not care how you say it; the bytes still begin GIF87a or GIF89a.

Why .gif files still sit on disks in 2026

In 2026, .gif legacy files still appear in Slack exports, email signatures, old marketing ZIPs, GeoCities-era mirrors, game UI packs, and phone camera “live” exports saved as loops. They remain easy to open in browsers and Photos when the stream is ordinary. The pain is elsewhere: uploaders that reject animation, printers that expect a still, and archives that want PNG or JPG for consistency.

Legacy File Converter lists Graphics Interchange Format / .gif among supported images on Supported File Types and converts .gif images locally on Windows to JPG, PNG, or PDF — nothing is uploaded. This article stays the history — not a how-to for building Application Extensions. For neighboring image-format lessons, see History Lesson: What Is a .bmp File?, History Lesson: What Is a .jfif File?, History Lesson: What Is a .tga File?, and History Lesson: What Is an .avif File?.