Is éard is séimhiú ann sa teanga scríofa ná h a chur i ndiaidh consain agus iad araon a fhuaimniú le chéile mar chonsan amháin. Is iad na consain atá i gceist, b, c, d, f, g, m, p, s, t. (Lenition in the written language is an h which follows a consonant and both pronounced together as one consonant. The consonants in question are, b, c, d, f, g, m, p, s, t.)
Ní féidir séimhiú a dhéanamh ar na consain eile ná ar s roimh c, f, m, p, t, v. (The other consonants can not be lenited, nor can s before c, f, m, p, t, v. )