In 1909, Jones wrote the short Pronunciation of English, a book which he later radically revised. The Outline of English Phonetics which followed in 1918 is the first truly comprehensive description of British Received Pronunciation, and indeed the first such description of the standard pronunciation of any language.
The year 1917 was a landmark in many ways. Jones became the first linguist in the western world to use the term phoneme in its current sense, employing the word in his article The phonetic structure of the Sechuana Language . Jones had made an earlier notable attempt at a pronunciation dictionary but it was now that he produced the first edition of his famous English Pronouncing Dictionary , a work which in revised form is still in print. It was here that the cardinal vowel diagram made a first appearance .
The problem of the phonetic description of vowels is of long standing, going back to the era of the ancient Indian linguists. Three nineteenth century British phoneticians deserve mention. Alexander Melville Bell (1867) devised an ingenious iconic phonetic alphabet which included an elaborate system for vowels. Alexander Ellis had also suggested vowel symbols for his phonetic alphabets. Henry Sweet did much work on the systematic description of vowels, coming up with what must be considered a somewhat over-elaborate system of vowel description involving a multitude of symbols. Jones however was the one who is generally credited with having gone much of the way towards a practical solution through his scheme of 'Cardinal Vowels', a relatively simple system of reference vowels which for many years has been taught systematically to students within the British tradition. It is worth pointing out, however, that much of the inspiration for this scheme can be found in the earlier publications of Paul Passy.
In the original form of the Cardinal Vowels, Jones employed a dual-parameter system of description based on the supposed height of the tongue arch together with the shape of the lips. This he reduced to a simple quadrilateral diagram which could be used to help visualize how vowels are articulated. Tongue height (close vs. open) is represented on the vertical axis and front vs. back on the horizontal axis indicates the portion of the tongue raised on the horizontal axis. Lip-rounding is also built into the system, so that front vowels (such as [i, e, a]) have spread or neutral lip postures, but the back vowels (such as [o,u]) have more marked lip-rounding as vowel height increases. Jones thus arrived at a set of eight "primary Cardinal Vowels", and recorded these on gramophone disc for HMV in 1917.
Later modifications to his theory allowed for an additional set of eight "secondary Cardinal Vowels" with reverse lip shapes, permitting the representation of eight secondary cardinal vowels (front rounded and back unrounded). Eventually Jones also devised symbols for central vowels and positioned these on the vowel diagram. He made two further disc recordings for Linguaphone in 1943 and 1956.
With the passing years, the accuracy of many of Jones's statements on vowels has come increasingly under question, and most linguists now consider that the vowel quadrilateral must be viewed as a way of representing auditory space in visual form, rather than the tightly defined articulatory scheme envisaged by Jones. Nevertheless, the International Phonetic Association still uses a version of Jones's model, and includes a Jones-type vowel diagram on its influential International Phonetic Alphabet leaflet contained in the "Handbook of the International Association". Many phoneticians (especially those trained in the British school) resort to it constantly as a quick and convenient form of reference.
Although Jones is especially remembered for his work on the phonetics and phonology of English, he ranged far more widely. He produced phonetic/phonolological treatments which were masterly for their time on the sound systems of Cantonese, Tswana (Sechuana as its was then known), Sinhalese, and Russian. He was the first phonetician to produce, in his "Sechuana Reader", a competent description of an African tone language, including the concept of downstep. Jones helped develop new alphabets for African languages, and suggested systems of romanisation for Indian languages and Japanese. He also busied himself with support for revised spelling for English through the Simplified Spelling Society.
Apart from his own vast array of published work, Jones will be remembered for having acted as mentor to numerous scholars who later went on to become famous linguists in their own right. These included such names as Lilias Armstrong, Harold Palmer, Ida Ward, Hélène Coustenoble, Arthur Lloyd James, Dennis Fry, A.C. Gimson, Gordon Arnold, J.D. O'Connor, and many more. For several decades his department at University College was pivotal in the development of phonetics and in making its findings known to the wider world. A point of interest is that it is probably Daniel Jones (and not as is often thought Henry Sweet) who provided George Bernard Shaw with the basis for his fictional character Henry Higgins in "Pygmalion". See the discussion in "The Real Professor Higgins" .