Which GIF file block stores a frame's animation delay?

The Graphic Control Extension stores a GIF frame’s animation delay. Each frame can have its own GCE, which tells a decoder how long to wait before advancing to the next frame.

The delay value is measured in hundredths of a second. For example, a stored value of 9 represents a nominal delay of 0.09 seconds. The GCE also contains other frame-level controls, including disposal instructions, a transparency flag, and a transparent color index.

The extension was added with GIF89a, released in 1989. GIF originally supported multiple images in one file, but GIF89a supplied the controls that made those images practical as animation frames. A frame’s GCE normally appears immediately before its Image Descriptor and image data.

The other options describe different blocks. The Image Descriptor defines a frame’s position and dimensions; the Logical Screen Descriptor describes the overall canvas; and an Application Extension can carry application-specific information, such as Netscape’s looping instructions. Thus, the Application Extension can control repetition, but it does not store the per-frame delay.