What unusual features are found in the Insular Celtic languages?

The Insular Celtic languages—comprising the Goidelic (Irish, Scottish Gaelic, Manx) and Brythonic (Welsh, Cornish, Breton) groups—exhibit several distinctive linguistic features.123

Phonological Characteristics and Mutations

The most notable feature is the development of a double series of consonants, distinguishing strong and weak articulations. Over time, these evolved into a system of initial mutations, where the initial consonant of a word changes based on the preceding word's influence. This occurred because consonant weakening, which typically happens between vowels within a word, extended to the phrase level and persisted even after final syllables were lost in the evolution of these languages.4

Other Notable Features

• Initial Verb Position: While once considered exotic, the placement of the verb at the beginning of a sentence is now understood as an organic development from Indo-European roots.5
• Vigesimal Counting: These languages utilize a system of counting by 20s, an innovation also found in other languages like Basque and certain forms of English and French.5

Some scholars suggest these traits may stem from a non-Celtic substratum in the British Isles, though this remains a subject of academic discussion.5