Lempel-Ziv 77 (LZ77) algorithm is the first Lempel-Ziv compression algorithm for sequential data compression. The dictionary is a portion of the previously encoded sequence. The encoder examines the input sequence through a sliding window as shown in Figure 9. The window consists of two parts:

1. a search buffer that contains a portion of the recently encoded sequence, and
2. a look-ahead buffer that contains the next portion of the sequence to be encoded.

In the diagram below, the search buffer contains eight symbols (in blue), while the look-ahead buffer contains nine symbols (in red). In practice, the sizes of the buffers are significantly larger.