Finally, the two groups differ radically in the matter of initial
mutation or, as it is often called, aspiration. These mutations are by
no means confined to initial consonants, as precisely the same changes
have taken place under similar conditions in the interior of words. The
Goidelic changes included under this head probably took place for the
most part between the 5th and 7th centuries, whilst in Brythonic the
process seems to have begun and continued later. It is easier to fix the
date of the changes in Brythonic than in Goidelic, as a number of
British names are preserved in lives of saints, and it is possible to
draw conclusions from the shape that British place-names assumed in the
mouths of the Anglo-Saxons. In Goidelic, we find two mutations, the
vocalic and the nasal. Initial mutation only takes place between words
which belong together syntactically, and which form one single
stress-group, thus between article, numeral, possessive pronoun or
preposition, and a following substantive; between a verbal prefix and
the verb itself.

1. When the word causing mutation ended in a vowel we get the vocalic
mutation, called by Irish grammarians aspiration. The sounds affected
are the tenues k (c), t, p; the mediae g, d, b; the liquids and nasals
m, n, r, l, s, and Prim. Celt. v (Ir. f, W. gw). At the present day
the results of this mutation in Irish and Welsh may be tabulated as
follows. Where the sound is at variance with the traditional
orthography, the latter is given in brackets. In the case of n, r, l
in Goidelic we get a different variety of n, r, l sound. In Welsh in
the case of r, l, the absolute initial is a voiceless r, l written rh,
ll, which on mutation become voiced and are written r, l. In Irish s
becomes h written sh and the mutation of f is written fh, which,
however, is now silent. Examples:--Irish, cu, "hound," _do chu_, "thy
hound"; Welsh ci, dy gi (do, dy represent a Prim. Celt. _*tovo_);
Irish _mathair_, "mother," _an mhathair_, "the mother," Welsh _mam, y
fam_ (the feminine of the article was originally _*senta, senda)._

+--------+---------+-------+-------+-------+-------+---------+---------+
|Original| | | | | | | |
| sound | k | t | p | g | d | b | m |
+--------+---------+-------+-------+-------+-------+---------+---------+
| Irish |[chi](ch)| h(th) | f(ph) |