Letter frequency xz. Modern International Morse code (generally believed to have been developed by Alfred Vail based on English-language letter frequencies of the 1830s) encodes the most frequent letters with the shortest symbols; arranging the Morse alphabet into groups of letters that require equal amounts of time to transmit, and then sorting these groups in increasing order, yields e it san hurdm wgvlfbk opxcz jyq. Similar ideas are used in modern data-compression techniques such as Huffman coding. Letter frequency was used by other telegraph systems, such as the Murray Code. Letter frequencies, like word frequencies, tend to vary, both by writer and
Prosigns for Morse code procedure word "ALL AFTER", and is used to indicate that part of the previously transmitted message needs to be re-transmitted; the only difference between the Morse code prosigns is an inter-letter space between the two "dot dash dot dash" sequences". Because there are no letter boundaries in the transmitted prosigns, their division into letters is arbitrary and may be done in multiple equivalent ways. For example, ' (+) is exactly equivalent to ' (+) and ' (+). Likewise, the well-known prosign ' could just as well be written ' (++), ' (++), or even (+). Normally, one particular form is