Cornish grammar is the grammar of the Cornish language (Kernowek or Kernewek), an insular Celtic language closely related to Breton and Welsh and, to a lesser extent, to Irish, Manx and Scottish Gaelic. It was the main medium of communication of the Cornish people for much of their history until the 17th century, when a language shift occurred in favour of English. A revival, however, started in 1904, with the publication of A Handbook of the Cornish Language, by Henry Jenner, and since then there has been a growing interest in the language.