The ordinal number cyntaf ('first') takes a treiglad meddal when it follows a feminine noun:
y wers gyntaf ar y diwrnod cyntaf the first lesson on the first day
gwers noun (f); diwrnod noun (m);
The ordinal number ail ('second') causes a soft mutation in both masculine and feminine nouns:
yr ail wers ar yr ail ddiwrnod the second lesson on the second day
Other ordinal numbers cause a treiglad meddal only for feminine nouns:
y seithfed ddinas, y seithfed pentref the seventh city, the seventh village
The ordinal numbers may be used on their own without a following noun, for example: 'the sixth', 'the tenth'. In this case, a treiglad meddal is applied if the number applies to a feminine person or object:
Y ferch oedd y ddegfed i ymuno â'r clwb.
The girl was the tenth to join the club.
but
Y bachgen oedd y degfed i ymuno â'r clwb.
The boy was the tenth to join the club.
When units of measurement follow the numbers two, three and four, the masculine or femine form is used depending on the gender of the unit:
tri munud, tair awr, tri diwrnod, tair wythnos, tri mis, tair blynedd
munud noun (m); awr noun (f); diwrnod noun (m);
wythnos noun (f); mis noun (m); blynedd noun (f);
tair punt pedair ceiniog punt noun (f) ; ceiniog noun (f);
tair troedfedd pedair modfedd troedfedd noun (f) ; modfedd noun (f);
but
tri metr pedwar centimetr metr noun (m) ; centimetr noun (m);