Irish consonants come in pairs consisting of a ‘slender’ version and a ‘broad’ version. For example, the ‘slender’ b is the first sound in the word beo (‘alive’), whereas the first sound in the word bó (‘cow’) is the ‘broad’ b. Beo and bó differ only in the slenderness vs. broadness of the b.
In phonetic terms, both the slender and the broad consonant of a pair are articulated with the same primary gesture. For b, this primary gesture is the lips closing to stop the airflow from the mouth. However, they differ in their secondary articulation — the details of how they are pronounced.
The slender consonants like the b in beo are palatalized, meaning that the tongue body moves up and forward toward the hard palate, as if making a “y” [j] sound, while the primary articulatory gesture is going on. (This is known as secondary palatalization. A consonant with a palatal primary articulatory gesture is English y (IPA /j/) as in you.)
In Irish, palatalized consonants are spelled with ‘i’ or ‘e’ next to them. In linguistics, they are written with a small ‘j’: [bʲ]. In the SSANOVA diagrams on this website, palatalized consonants are represented with green lines. (See Interpreting Figures.)
The broad consonants, in contrast, are velarized. The term velarized refers to the velum, or soft palate, toward the back of the mouth. When a velarized consonant like the b in bó is pronounced, the tongue body moves back and up toward the velum. The velum is the place of primary articulation for consonants like /k/ in kangaroo and /g/ in gorilla.
In Irish spelling, velarized consonants are written with back vowels (u, o, a) next to them (e.g., ‘b’ in bó). In the IPA, they are written with the symbol for the voiced velar fricative: [bˠ]. Our diagrams represent the velarized consonants with black lines. (See Interpreting Figures.)