Welsh units are the historical units of measurement in Wales separate from those imposed by the English following their 13th-century Edwardian conquest. Modern Wales no longer employs any of these but – like the rest of the UK – instead uses a mixture of metric and British imperial units.
In the Laws of Hywel Dda:
- 2 rods × 18 rods = 1 erw (acra)
- 312 erwaw = 1 rhandir
- 3 rhandiroedd (held by taeogion) = 1 taeogtref
- 4 rhandiroedd (held in freehold) = 1 tref ryd
- 7 taeogtrefi = 1 maenor vro = 936 erwau
- 12 trefi ryd = 1 maenor wrthdir = 1248 erwau 
The cymydau and cantrefi, meanwhile, were fixed political entities with quite various sizes.
Subsequently, at least in theory:
- 4 erwau = 1 tyddin (homestead)
- 4 tyddynnod = 1 rhandir (tract, district)
- 4 rhandiroedd = 1 gafael (holding)
- 4 gafelion = 1 tref (township)
- 4 trefi = 1 maenor (manor) = 1024 erwau
- 12½ maenorau = 1 cwmwd (commote)
- 2 cymydau = 1 cantref = 25,600 erwau
- 1 Hestawr = 2 Winchester bushels
- Lewis, Timothy. A glossary of mediaeval Welsh law, based upon the Black book of Chirk. Univ. Press (Manchester), 1913.
- One version of the Laws of Hywel Dda, the Latin Peniarth MS. 28, instead gives 16½ feet to the 'long yoke'.
- Wade-Evans, Arthur. Welsh Medieval Law, p. 339. Oxford Univ., 1909. Accessed 1 Feb 2013.
- Wade-Evans. p. 344
- Lewis's account, based on Gwynedd's Black Book of Chirk, gives the gafael as holding 34 erwau rather than 64.
- Williams, Jane. A History of Wales. Cambridge Univ, 2010. Accessed 1 Feb 2013.
- The modern Welsh word for "week" is wythnos: "eight nights"
- Wade-Evans, Arthur. Welsh Medieval Law, n. 5 by Mary Jones. Accessed 1 Feb 2013.
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