Most of the place names in the Upper Usk Valley are Welsh, naturally enough. Many of them go back a considerable time, and quite a few of them have changed through the years due to such factors as mutation (a feature of Welsh grammar), Anglicisation, and a lack of standardisation of spelling in years gone by. This appendix provides a key to the translations of the usually-accepted names. It is worth including because many of the names are of ancient origin and they can provide an insight into topography which may have changed over the years.

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• Aberhenwen
  Mouth of the Henwen Brook (= 'fair old' brook)
• Allt-y-Fan
  Wood or slopes of the peak
• Blaenwysg
  Head of the Usk River
• Brynmaendy
  Hill of black stones
• Castell Du
  Black or dark castle
• Cwmwysg
  Valley of the Usk
• Dorallt
  Break of mountain slopes
• Ffôs Ddu
  Black or dark ditch
• Ffosyhywaid
  Ditch of the ducks
• Gelligam
  Crooked wood
• Gwernwyddog
  Alder trees or wooded marsh
• Llwyn-celyn
  Holly bush
• Llwyn-Cor
  Grove of song
• Llwynmeurig
  Meurig's grove
• Llwyn-On
  Grove of the ash trees
• Llywel
  Church of (St.) Llywel
• Nant-y-Pistyll
  Brook of the spring
• Penrhiw
  Top of the slope
• Pentrebach
  Small village
• Pentwyn
  Top of the hill
• Pen-y-Cae
  End of the field
• Pwll Isaf
  Lower pool
• Pwll Uchaf
  Upper pool
• Tai-Newydd
  New houses
• Traen-Glas
  Green land
• Traen-Mawr
  Great land
• Trawslywn-Du
  Black cross hedge
• Tre-castell
  Town of the castle
• Tyle-Mawr
  Big hillside
• Ynysclydach
  Meadow of Clydach (stream)
• Ynysfain
  Narrow meadow
• Ynysmarchog
  Knight's meadow