In GIF file structure, the ASCII character comma introduces an Image Descriptor.
The comma is represented by hexadecimal byte 0x2C in a GIF data stream. After the decoder encounters this sentinel, it reads the fixed-size Image Descriptor fields, including the image’s left and top position, width, height, and packed flags. Optional local color-table data may follow before the image’s LZW-compressed pixel data.
A GIF can contain multiple images, and each image begins with its own 0x2C marker. In an animated GIF, those images commonly serve as frames, often preceded by a Graphic Control Extension that supplies timing, transparency, or disposal instructions. The Image Descriptor itself describes geometry and image-related flags, not the animation delay.
The other structural punctuation marks are common mix-ups. 0x21, the ASCII exclamation point, introduces an extension block; 0x3B, the semicolon, marks the trailer; and the colon is not the Image Descriptor sentinel. Thinking of 0x2C as the separator that announces the next image is a useful way to remember its role.