Glottochronology  Glottochronology (from Attic Greek γλῶττα "tongue, language" and χρóνος "time") is the part of lexicostatistics dealing with the chronological relationship between languages. The idea was developed by Morris Swadesh under two assumptions: there indeed exists a relatively stable "basic vocabulary" (referred to as "Swadesh lists") in all languages of the world; and, any replacements happen in a way analogous to radioactive decay in a constant percentage per time elapsed. Over time many different extensions of the Swadesh method evolved; however, Swadesh's original method is so well known that 'glottochronology' is usually associated with him. The original method presumed that the core vocabulary of a language is replaced at a constant (or constant average) rate across all languages and cultures and so can be used to measure the passage of time. The process makes use of a list of lexical terms. Lists were compiled by Morris Swadesh and assumed to be resistant against borrowing (originally designed in 1952 as a list of 200 items, but the refined 100-word list in Swadesh (1955) is much more common among modern day linguists). The core vocabulary was designed to encompass concepts common to every human language (such as personal pronouns, body parts, heavenly bodies, verbs of basic actions, numerals 'one' and 'two'), eliminating concepts that are specific to a particular culture or time. It has been found that the ideal is really impossible and that the meaning set may need to be tailored to the languages being compared. Many alternative word lists have been compiled by other linguists and often use fewer meaning slots. The percentage of cognates (words with a common origin) in the word lists is then measured. The larger the percentage of cognates, the more recently the two languages being compared are presumed to have separated. Robert Lees obtained a value for the "glottochronological constant" (r) of words by considering the known changes in 13 pairs of languages using the 200 word list. He obtained a value of 0.805 ± 0.0176 with 90% confidence. For his 100-word list Swadesh obtained a value of 0.86, the higher value reflecting the elimination of semantically unstable words. The constant is related to the retention rate of words by the following formula:  "L" is the rate of replacement, ln represents the natural logarithm and "r" is the glottochronological constant.