It is sometimes wrongly claimed that the name Tredegar can be explained as tre deg erw, an adaptation and reduction of "tref y deg erw" or reduction of "tre'r deg erw" ("tref" is an older form of "tre"; "y" is the definite article after a consonant, and "'r" is the definite article after a vowel; in either case in place names the linking definite article is often dropped, hence pairs such as "Glan-y-môr, Glan-môr" (the sea's edge) or "Cae'r Maen, Cae Maen" (the field of the standing stone). Deg erw is Welsh for "ten acres"; "tre" in newer place names in the industrialised valleys means "town", and equates to English names with "town" (usually a grand name for rows of workers' housing) (e.g. Hopkinstown, Rhondda) or "ville" (e.g. Edwardsville, Merthyr Tudful). Historically "tref / tre" was approximately a "homestead, farmstead, hamlet, estate". In this respect we can compare the sense development of Old English "tūn" (= farmstead) to modern English "town". So "tre deg erw" is plausible morphologically, but is not the origin of the name "Tredegar".
Another erroneous explanation, which was also around in the 1800s, is that Tredegar is tri deg erw, which means "thirty acres".
In the case of both "ten acres" and "thirty acres" there is no indication of what this land area might refer to. In the second case, "tri deg erw" could not have resulted in "Tredegar".
In both of the above interpretations it is supposed that "erw" has been reduced to "er" through the loss of the final vowel "w", and the resulting final syllable "er" has become final "ar". This would be consistent with features of south-eastern Welsh, or Gwentian, which is the variety of Welsh spoken historically in Tredegar. South-eastern field names show this reduction – Dwyar, a field name in Penderyn (dwy erw = two acres > dwyer > dwyar).
The resulting form would be Tredegar, but this supposes that this is an altered form of tri-deg-ar. "Tri-deg" (three tens) is thirty, but as a numeral is a recent innovation in Welsh, since "deg-ar-hugain" (ten on twenty) is the traditional numeral. In addition, "tri-deg" would hardly change to "tre-deg".
In fact, the town of Tredegar is so called because in 1800 Samuel Homfray, who had married into the Morgan of Tredegar family, formed a company to produce iron which was named the Tredegar Iron Company – the land where he extracted and treated the ore belonged to his father-in-law and was a part of the Tredegar Estate. The company’s buildings appeared on an 1832 Ordnance Survey map as Tredegar Iron Works.
The Tredegar in the name of the company and its ironworks referred to the original Tredegar, which is in Coedcernyw by Newport, and is nowadays more usually known in English as (in order to avoid confusion) Tredegar House (or Tredegar Park). Older forms of the name show it to be Tredegyr (this form is found in 1550) (by the modern Welsh period generally this final "y" would have become "e", and in Gwentian this would have in turn become "a", as with Gwentian "Merchar" (Wednesday), standard Welsh "Mercher", from older Welsh "Merchyr").