Along with GIF, XBM was one of the first two common image formats used on websites. XBM stands for X BitMap and was a simple black-and-white bitmap format associated with the X Window System.
XBM files represented images as text-based source code, typically as arrays of hexadecimal values. Because browsers could interpret the data easily and the format required no specialized binary decoder, XBM was useful for small icons, buttons, and other stark interface graphics on early Unix-oriented systems.
GIF served a different role. It supported color palettes and, in its later GIF89a version, features such as animation delays and transparent background colors. That made GIF much more visually versatile than XBM, especially as websites began using decorative graphics and animated images.
PNG, JPEG, and BMP are common modern or historical image formats, but they are not the specific black-and-white companion identified in this early-Web account. XBM eventually became a legacy format as browsers and websites adopted richer, more compact, and more widely supported alternatives.