It appears that Cadair/Cadeir Idris is the form used in the earliest Welsh-language sources. In a poem in his own hand in the second half of the 15th century, the poet Lewys Glyn Cothi wrote 'Dros gadair idris gedy' ('and then over Cadair Idris'). Around 1600, John Jones of Gellilyfdy referred to 'y mynydh neu bhan neu bhoel a elwir Cadeir Idris' ('the mountain, peak or hill known as Cadeir Idris').