e.g. xiążę, xięstwo (now książę, księstwo). Later was replaced by and in almost all words and remained only in a few loanwords as 'xenia' (xenien), surnames as Axentowicz, Jaxa, Koxowski, Mixtacki, Rexemowski, Xiężopolski, names as Xawery, Xymena and abbreviations. Additionally, in languages for which the Latin alphabet has been adapted only recently, has been used for various sounds, in some cases inspired by European usage, but in others, for consonants uncommon in Europe. For these no Latin letter stands out as an obvious choice, and since most of the various European pronunciations of can be written by other means, the letter becomes available for more unusual sounds. represents (voiceless velar fricative) in e.g. Azerbaijani, Kurdish (Hawar alphabet), Georgian (when Latinized), Lojban, Pashto (when Latinized), Tatar (Jaꞑalif, Zamanälif, official romanization of 2012), Uzbek, and Uyghur (Latin script). Esperanto: The x-convention replaces , , , , , and with x-suffixes: , , , , , and . In transliteration of Indian languages, primarily Indo-Aryan languages, represents the consonant cluster in alternate spellings of words containing 'क्ष' (kṣ), especially names such as Laxmi and Dixit. Less frequently, is used to represent 'ख़' . In Apache represents In Nahuatl, represents . In Nguni languages, represents the alveolar lateral click . In Pirahã, symbolizes the glottal stop . An illustrating example of "x" as a "leftover" letter is differing usage in three different Cushitic languages: Afar: voiced alveolar implosive Oromo: alveolar ejective Somali: voiceless pharyngeal fricative In East and Southeast Asia: In Lao, based on romanization of Lao consonants, may represent , e.g. in Lan Xang. In Vietnamese, is pronounced like English (at the beginning of a word, e.g. "sing"). This sound was in Middle Vietnamese, resembling the Portuguese sound , spelled . In Hanyu Pinyin, Standard Chinese's official transcription system in China, Malaysia, Singapore, and Taiwan, the letter represents the voiceless alveolo-palatal fricative , for instance in 'Xi', . Other systems In the International Phonetic Alphabet, represents a voiceless velar fricative. Other uses In mathematics, x is commonly used as the name for an independent variable or unknown value. The modern tradition of using x, y and z to represent an unknown (incognita) was introduced by René Descartes in La Géométrie (1637). As a result of its use in algebra, X is often used to represent unknowns in other circumstances (e.g. X-rays, Generation X, The X-Files, and The Man from Planet X; see also