For example, no one would mistake xwllygrZPXll mRRtQzryWx for a portion of normal text. It is possible to make use of the fact that certain sequences of letters are more common than others to fit a text document, or a data file of another type, into less than the usual amount of space. Graphics and audio files can be compressed by specialized techniques. Other, more specialized techniques of compression for such files involve Fourier transforms. The most popular such technique is the Discrete Cosine Transform, used to produce JPEG files.
Wavelet compression is another related technique.
A very popular data compression technique is Lempel-Ziv compression. This is used with general purpose file compression programs on computers. Its basic principle is that, as a file is read, the part of the file that has already been processed is used as a dictionary of sequences of bytes likely to occur in that file. When a repeated sequence of bytes is long enough, a pointer back to the earlier part of the file is shorter than repeating the bytes. Huffman coding is the other major data compression technique applicable to text files.