In order for steganography to find a handle in digital data files, the format of those files must contain a degree of non-predictable variation. Steganography operates by substituting desired bit values in unpatterned bit positions. Fortunately, many file formats contain quite a bit of non-predictable variation. The most commonly-used file formats for steganography are those that encode real-world (analog) data, such as image and sound formats. Typically, a subchannel in an image is encoded in the "least significant bits" of that image. That is, if each image's pixel color is encoded with a number of bits (often 24) some of those bits cause less color variation of the pixel than others do. Specifically, 24-bit images usually have 8-bit values devoted to each primary color (red, green, blue). If the image is generated through a real-world process (such as taking a photograph), the sequence of lowest-order red bits will be largely random to start with (because of the finite resolution of cameras and also because of "random" variations in the pictured object). A steganographic encoding might substitute subchannel values into that sequence of lowest order red bits (red variation is the least perceptible of the primary colors). The receiver reads the subchannel back out of a received image by stripping out everything other than the sequence of lowest order red bits (which are identified purely positionally by the file-format structure).