The names "Memelburg" and "Memel" are found in most written sources from the 13th century onwards, while "Klaipėda" is found in Lithuania-related sources since the 15th century. The first time the city was mentioned as "Caloypede" in the letter of Vytautas in 1413, for the second time in the negotiation documents of 1420 as "Klawppeda", and for the third time in the Treaty of Melno of 1422 as "Cleupeda". According to Samogitian folk etymology, the name "Klaipėda" refers to the boggy terrain of the town ("klaidyti"=obstruct and "pėda"=foot). Most likely the name is of Curonian origin and means "even ground": "klais/klait" (flat, open, free) and "peda" (sole of the foot, ground), as a reference to relatively flat terrain of the original settlement's surroundings.