In the ninth century, new invaders from across the sea began to appear on England's
eastern coast--the Vikings, or Danes, as the English called them. Decades of sporadic violence followed.
After a decisive defeat by the English king Alfred, the Danes agreed by treaty to stay northeast
of a line running diagonally across Britain, called the Danelaw.
As generations passed, the interaction of the two cultures greatly influenced the development of English. Since Danish and Old English were both Germanic languages, they were similar from the start. Important changes in English occurred as the Danish and the English began to trade with each other and intermarry, because they had to speak a simplified language in order to communicate.
The most important grammatical change was that English words lost many of their endings. Instead of its spelling, a word's place in a sentence now began to determine its meaning.
English also developed a richer vocabulary in this period. It absorbed new Danish words and Danish synonyms for English words. Place names in modern Great Britain that end in -by also are part of the Danish heritage.