 To spot matches, the encoder must keep track of some amount of the most recent data, such as the last 2 kB, 4 kB, or 32 kB.The structure in which this data is held is called a "sliding window", which is why LZ77 is sometimes called "sliding-window compression".The encoder needs to keep this data to look for matches, and the decoder needs to keep this data to interpret the matches the encoder refers to.