JPEG is the most widely used lossy compression standard for photographic still images. The name comes from the Joint Photographic Experts Group, the joint ISO/IEC and ITU-T committee that produced it. The core technical specification is ITU-T Recommendation T.81, titled “Information technology - Digital compression and coding of continuous-tone still images - Requirements and guidelines,” approved in September 1992 and published in identical form as ISO/IEC 10918-1. The ITU catalogue record lists T.81 (09/92) as the in-force version, produced through joint work with ISO and IEC.
The standard targets continuous-tone still images, meaning photographs and similar imagery where colors and brightness vary smoothly rather than in the sharp flat regions typical of line art and text. T.81 specifies several modes of operation. The baseline sequential mode, which is by far the most commonly implemented, uses the discrete cosine transform (DCT). The image is divided into eight-by-eight blocks of pixels, each block is transformed into a set of frequency coefficients, those coefficients are quantized (the lossy step, which discards visual detail the human eye is least sensitive to), and the quantized values are then entropy coded using Huffman coding.
Because quantization throws away information, JPEG is a lossy codec: the decompressed image is an approximation of the original, and the degree of loss is controlled by a quality setting that scales the quantization tables. Higher compression ratios produce smaller files at the cost of visible artifacts, especially the blocky “mosquito noise” and ringing that appear around sharp edges when the eight-by-eight block structure becomes apparent. The standard also defines progressive, hierarchical, and lossless modes, though these saw far less adoption than baseline DCT coding.
T.81 defines only the compressed data stream and the decoding process, not a file format. The familiar files on disk use container formats layered on top, most commonly JFIF (JPEG File Interchange Format) and later Exif, which add headers carrying resolution, color space, thumbnails, and camera metadata. This separation between the coded representation and its container is a recurring pattern in multimedia standards, and JPEG is one of its earliest mass-market examples.
JPEG arrived just as digital photography, scanners, and the early web were creating enormous demand for compact images, and its combination of strong compression and acceptable quality made it the default photographic format for decades. It coexists with lossless formats such as PNG and GIF, which are better suited to graphics, screenshots, and images requiring exact pixel reproduction. The committee continued to produce successor work, including JPEG 2000 and later wavelet-based and learning-based codecs, but the original DCT-based JPEG of T.81 remains one of the most deployed image standards in history.