ambiguity by providing a single letter for each phoneme: .Đ used to be commonly written as Dj on typewriters, but that practice led to too many ambiguities. It is also used on car license plates. Today Dj is often used again in place of Đ on the Internet as a replacement due to the lack of installed Serbo-Croat keyboard layouts. Montenegrin alphabet, adopted in 2009, provides replacements of and with digraphs and in both Latin and Cyrillic, but they remain largely unused, even by the Parliament of Montenegro which introduced them. Unicode has separate characters for the digraphs lj (Ǉ, ǈ, ǉ), nj (Ǌ, ǋ, ǌ) and dž (Ǆ, ǅ, ǆ). Dialects South Slavic historically formed a dialect continuum, i.e. each dialect has some similarities with the neighboring one, and differences grow with distance. However, migrations from the 16th to 18th centuries resulting from the spread of Ottoman Empire on the Balkans have caused large-scale population displacement that broke the dialect continuum into many geographical pockets. Migrations in the 20th century, primarily caused by urbanization and wars, also contributed to the reduction of dialectal differences. The primary dialects are named after the most common question word for what: Shtokavian uses the pronoun što or šta, Chakavian uses ča or ca, Kajkavian (kajkavski), kaj or kej. In native terminology they are referred to as nar(j)ečje, which would be equivalent of "group of dialects", whereas their many subdialects are referred to as dijalekti "dialects" or govori "speeches". The pluricentric Serbo-Croatian standard language and all four contemporary standard variants are based on the Eastern Herzegovinian subdialect of Neo-Shtokavian. Other dialects are not taught in schools or used by the state media. The Torlakian dialect is often added to the list, though sources usually note that it is a transitional dialect between Shtokavian and the Bulgaro-Macedonian dialects. The Serbo-Croatian dialects differ not only in the question word they are named after, but also heavily in phonology, accentuation and intonation, case endings and tense system (morphology) and basic vocabulary. In the past, Chakavian and Kajkavian dialects were spoken on a much larger territory, but have been replaced by Štokavian during the period of migrations caused by Ottoman Turkish conquest of the Balkans in the 15th and the 16th centuries. These migrations caused the koinéisation of the Shtokavian dialects, that used to form the West Shtokavian (more closer and transitional towards the neighbouring Chakavian and Kajkavian