F    - (a mutation of B and of M; e.g. "bach" becomes "fach" in "Gilfach", "Mair" becomes "fair" in "Llanfair")

Word element	Meaning	Example of use (sound file links underlined)
fach, fechan	see bach, bychan	Fforestfach; Llanfairfechan
faen	see maen
faenor	see maenor	Llanfaenor
faer	see maer
faes	see maes	Llanfaes
Fair	see Mair	Llanfair Caereinion
Faldwyn	see Maldwyn
fan	see ban	Aberfan
faerdref	see maerdref
fawnog	see mawnog
fawr	see mawr	Llanbadarn Fawr
fechan	see bach, bychan	Llanfairfechan
fedwen, fedw	see bedwen, bedw
felen	see melyn
felin	see melin	Felin-foel
Fellte	see Mellte	Ystradfellte
felys	see melys
fendigaid	see bendigaid	Pontrhydfendigaid
fer	short
Fihangel	see Mihangel	Llanfihangel-y-Creuddyn
foel	see moel	Felin-foel
fôr	see môr
forwyn	see morwyn
fraith	see brith
frân	see brân
fre	see bre
fron	see bron	Fron-goch
fryn	see bryn
fudr	see budr
fynydd	see mynydd	Trawsfynydd

  M    - (M can be a mutation of B, and "Mh" can be a mutation of P)

Word element	Meaning	Example of use (sound file links underlined)
maen, (plural meini)	stone	Penmaenmawr
maenol,maenor	residence of district chief
maerdre(f)	hamlet attached to chief's court, lord's demesne
maes, (plural meysydd)	field, plain
Mair	saint's name, Welsh equivalent of Mary	Llanfair Caereinion
mawr	great, big	Penmaenmawr
meini	see maen
mêl	honey	Nantmel
melin	mill	Melincwrt
melindre(f)	mill village
melyn	yellow
merthyr	burial place, church	Merthyr Tudful (Merthyr Tydfil)
mign, (plural mignedd)	bog, mire
moel	bare hill, bald
morfa	marsh, sea fen
mur, (plural muriau)	wall
mwyn	ore, mine
mynachlog	monastery
mynydd	mountain, moorland	Mynydd Du