correspond to the Greek numerals, with being used for digamma, for koppa, and for sampi. The system was abandoned for secular purposes in 1708, after a transitional period of a century or so; it continues to be used in Church Slavonic, while general Russian texts use Indo-Arabic numerals and Roman numerals. Diacritics The Cyrillic alphabet and Russian spelling generally employ fewer diacritics than those used in other European languages written with the Latin alphabet. The only diacritic, in the proper sense, is the acute accent (Russian: 'mark of stress'), which marks stress on a vowel, as it is done in Spanish and Greek. (Unicode has no code points for the accented letters; they are instead produced by suffixing the unaccented letter with .) Although Russian word stress is often unpredictable and can fall on different syllables in different forms of the same word, the diacritic accent is used only in dictionaries, children's books, resources for foreign-language learners, the defining entry (in bold) in articles on Russian Wikipedia, or on minimal pairs distinguished only by stress (for instance, 'castle' vs. 'lock'). Rarely, it is also used to specify the stress in uncommon foreign words, and in poems with unusual stress used to fit the meter. The letter is a special variant of the letter , which is not always distinguished in written Russian, but the umlaut-like sign has no other uses. Stress on this letter is never marked with a diacritic, as it is always stressed (except in some compounds and loanwords). Both and the letter have completely separated from and . has been used since the 16th century (except that it was removed in 1708, but reinstated in 1735). Since then, its usage has been mandatory. It was formerly considered a diacriticized letter, but in the 20th century, it came to be considered a separate letter of the Russian alphabet. It was classified as a "semivowel" by 19th- and 20th-century grammarians but since the 1970s, it has been considered a consonant letter. Frequency The frequency of characters in a corpus of written Russian was found to be as follows: Keyboard layout The standard Russian keyboard layout for personal computers is as follows: However, there are several variations of so-called "phonetic keyboards" that are often used by non-Russians, where, as far as is possible, pressing an English letter key will type the Russian letter with a similar sound (A → А,