Abstract
The article discusses the use of harmony, the fundamental rule inherent in Turkic languages, which is the basis for word formation, in the work of Mahmud al-Kashgari «Diwani lugat it-Turk». The conclusions made about the phonological function of harmony in Turkology are discussed, and its main word-forming, prosodic function is analyzed. The works of Mahmud al-Kashgari proves that the harmony rule was widely used in the language of the Turkic peoples in the Middle Ages. By using the rule while speaking, it makes words to be convenient to pronounce and sound melodious. The article shows that affixes follow the harmony when appending to a word, that the rule is preserved in word formation and examples are given. When compiling the dictionary, Mahmud Kashgari pays attention to how many letters(sounds) words consist of, grouping them into words consisting of two, three, four, five, six, seven letters. If we take a look at the grouped words, every group follows the palatal harmony and the labial harmony. From this work that was written in the Middle Ages, it is proved that during this period the harmony rules were widely used and well established as the main characteristics of the Turkic languages.