JPEGs use Huffman coding to convert a series of bytes (DCT coefficients) into a series of bits. Huffman coding is optimal if each member of the input distributi

JPEGs use Huffman coding to convert a series of bytes (DCT coefficients) into a series of bits. Huffman coding is optimal if each member of the input distribution has a 1/2^n probability (e.g., if 1/2 of the input is 0, you could represent that with just one zero bit). Arithmetic coding allows fractional probability distributions to be represented. It's usually at least 10% more efficient than Huffman coding.