In his Dictionarum Duplex published in 1632, John Davies of Mallwyd differentiated between the two words cadair ('chair') and cader ('fort, fortress'), referring to 'Cader Idris' and 'Cader Ddinmael'. That interpretation was followed by some later lexicographers, including Thomas Charles and Titus Lewis. It is now accepted, however, that cader is a spelling of a spoken form of cadair, and that there is no evidence that cadair and cader are separate words. But as it represents the pronunciation in the local dialect, the spelling Cader Idris is often seen in Welsh and English, and in June 2016 Snowdonia National Park decided to adopt that spelling on its signage, despite advice from the Welsh Language Commissioner and from Park officers, who favoured 'Cadair'. The local dialect form is also seen in the name of the local secondary school, Ysgol y Gader. The name of the mountain is typically spelt Cadair Idris on current maps. The summit of the mountain is known as Penygader (top of the chair/stronghold).