If you use all 8 bits of a byte for data, you need a separate way to store the length, or you need to make everything fixed-width which wastes space. "Varints"

If you use all 8 bits of a byte for data, you need a separate way to store the length, or you need to make everything fixed-width which wastes space. "Varints" or variable-length integers, with 7 bits of data and 1 continuation bit per byte, are used in a fairly wide variety of formats (e.g. Protobuf, SQLite and Lucene). They're compact when most of your integers are small, while still being quite simple and fast to encode/decode. Faster alternatives tend to be more complicated, and more compact alternatives tend to be slower. See https://en.wikipedia.org/wiki/Variable-length_quantity https://en.wikipedia.org/wiki/Variable-length_quantity