alongside Russian cosmonauts usually take Russian language courses. This practice goes back to the Apollo–Soyuz mission, which first flew in 1975. In March 2013, Russian was found to be the second-most used language on websites after English. Russian was the language of 5.9% of all websites, slightly ahead of German and far behind English (54.7%). Russian was used not only on 89.8% of .ru sites, but also on 88.7% of sites with the former Soviet Union domain .su. Websites in former Soviet Union member states also used high levels of Russian: 79.0% in Ukraine, 86.9% in Belarus, 84.0% in Kazakhstan, 79.6% in Uzbekistan, 75.9% in Kyrgyzstan and 81.8% in Tajikistan. However, Russian was the sixth-most used language on the top 1,000 sites, behind English, Chinese, French, German, and Japanese. Dialects Russian is a rather homogeneous language, in dialectal variation, due to the early political centralization under Moscow's rule, compulsory education, mass migration from rural to urban areas in the 20th century, and other factors. The standard language is used in written and spoken form almost everywhere in the country, from Kaliningrad and Saint Petersburg in the West to Vladivostok and Petropavlovsk-Kamchatsky in the East, the enormous distance between notwithstanding. Despite leveling after 1900, especially in matters of vocabulary and phonetics, a number of dialects still exist in Russia. Some linguists divide the dialects of Russian into two primary regional groupings, "Northern" and "Southern", with Moscow lying on the zone of transition between the two. Others divide the language into three groupings, Northern, Central (or Middle), and Southern, with Moscow lying in the Central region. All dialects are also divided into two main chronological categories: the dialects of primary formation (the territory of the Grand Duchy of Moscow roughly consists of the modern Central and Northwestern Federal districts) and secondary formation (other territories where Russian was brought by migrants from primary formation territories or adopted by the local population). Dialectology within Russia recognizes dozens of smaller-scale variants. The dialects often show distinct and non-standard features of pronunciation and intonation, vocabulary, and grammar. Some of these are relics of ancient usage now completely discarded by the standard language. The Northern Russian dialects and those spoken along the Volga River typically pronounce unstressed clearly, a phenomenon called okanye (). Besides the absence of vowel reduction, some dialects have high or diphthongal in place of and in stressed closed syllables (as in Ukrainian) instead