1. For use in the transmission of facsimile data, a method of encoding and time compressing data taken from a document, the face of which may be considered as being made up of a plurality of different elemental areas each of which contains indicia of a first state or indicia of a second state, comprising the steps of: (a) sequentially examining the contents of each elemental area to identify alternating run lengths of indicia of said first and second states; (b) counting the number of elemental areas in each run length; and (c) converting the number counted for each run length into a code term including a first code word n-bits in length and capable of representing up to x elemental areas, and a series of further code words, each having a bit length which exceeds the bit length of the preceding code word by one bit, the number of said further code words required being determined by the extent to which the number counted exceeds the capacity of said first code word.