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The term Latgalians (Latgalian: latgalīši, latgali, Latvian: latgaļi, letgaļi, leti (also spelt Latgallians and sometimes known as Lettigalls, Latgolans, or Lettigallians) can refer to the inhabitants of the Latgale region in eastern Latvia in general, the ethnic Latvians of Latgale (especially those who speak the Latgalian language), or to the ancient Baltic tribe. In English, the more archaic Lettigallians usually refers to the tribe.
In Latvian, latgaļi refers to the ancient tribe, whilst latgalieši refers to the ethnic Latvians of Latgale, which developed separately from the rest of ethnic Latvia in 1621-1917, though the terms are confused in colloquial Latvian.
The ancient Latgalians 
The ancient Letts, or "Latgalians" (i.e. inhabitants of the primal Latgale, likely derived from *Lætgale = "The Lettland"; cf. *Zemgale = "The lower-land"), were a Central Baltic tribe whose origin is little known, who, in the 5th and 6th centuries, dwelt only in the eastern part of present-day Vidzeme (westwards from the Aiviekste River), and later on, in nearly all the territory of that region. In written sources, they are mentioned from the 11th century. In the first two decades of the 13th century, the (Western) Latgalians allied with German (mainly Saxon) crusaders, and their lands were incorporated under unclear circumstances into Livonia. In the 13th and 14th centuries, they assimilated, in present-day Latgale, an unknown East-Baltic tribe, who thus became a part of today's ethnic Latvians (the Latvian term latgalieši "Latgalians", in the sense of "Eastern Latvians", dates only to the early 20th century).
Archaeological data 
The most accurate source of information about the ancient (Western) Latgalians comes from archaeology. In the lands of the Western and the Eastern Latgalians, about 80 flat cemeteries of Western Latgalian origin have been excavated, with more than 2000 burials uncovered. The first large scale excavations took place in Ludza Odukalns Cemetery in Latgale (1890-1891), where 339 Late Iron Age burials were uncovered. In the excavations at Pildas Nukši Cemetery (in Latgale, 1947-1948), 218 burials were uncovered, dated to 9th-12th century, while at Zvirgzdenes Kivti Cemetery (in Latgale; 1948, 1955-1958) 175 burials from the 7th-12th century were excavated. 315 burials were found at Aglonas Kristapiņi Cemetery (in Latgale; 1928, 1938, 1977-1980, 1984-1987, 1999-2000), in use from the late 8th to 12th century. Ērgļu Jaunāķēni Cemetery was totally excavated in 1971-1972, when 89 burials were found. At Koknese Cemetery 102 burials from the Late Iron Age were uncovered (1986-1989). In the area of Gauja's Latgallians two cemeteries, Drabešu Liepiņas and Priekuļu Ģūģeri are well excavated too.
Archaeological excavations have also been carried out on the hillforts of Ķente, Koknese, Sārumkalns, Tanīskalns, as well as on other Latgalian sites. Only a few (Western) Latgalian settlements have been excavated; large scale excavations (1960s-1970s) and reconstruction has been done at the Āraiši lake dwelling site (9th century).
In Latgale, dating from the 6th-7th centuries, there were flat cemeteries as well as barrow cemeteries, but in the 9th-10th century the transition started from flat graves to barrows. There are about 15 excavated Eastern Latgalian barrow cemeteries, but usually only a small number of barrows were investigated.
Archaeologically identified dwelling-sites in Latgale include hill forts, settlements and lake dwellings. Among hillforts, well researched is Jersika Hillfort (excavated in 1939 and from 1990 onwards), forming complex together with Dignāja Hillfort, on the opposite bank of Daugava. Jersika was occupied in the 10th-14th century, probably after the decline of Dignāja, which had been inhabited since the 6th century.
It is possible that ancestors of the so-called Eastern Latgalians migrated to the territory of present-day Latgale in the 7th and 10th-11th centuries, pushed from their previously inhabited territory by Slavic migration; some archaeologists also believe that the Eastern Latgalians formed from the most ancient East-Baltic inhabitants of Latvia, who lived in central and Eastern parts of Latvia just in the "Roman" Iron age (1st-4th centuries).
In Latgalian, the terms latgalīši and latgali have been traditionally used as synonyms describing both the ancient and contemporary Latgalians as a part of the same continuum. The term latgalīši (in both meanings) prevailed in Latgalian literature and mass media during the first decades of the 20th century, whereas the term latgali has been consistently used (also in both meanings) in Latgalian literature and media published during the 1920s and 1930s in Latvia and from 1940 to 1988 in exile. Since the cultural revival of Latgalians in 1988, there is a tendency in Latgalian literature and media to follow Latvians in their use of both names.