The sound inventory of Proto-Northern Luzon shows no innovations from Proto-Malayo-Polynesian that would set it apart from other Philippine languages. There are however two phonological innovations that characterize the Northern Luzon languages:
Loss of final *ʔ (< *q)
Metathesis of *s and *t, e.g. Proto-Northern Luzon *saŋit < Proto-Malayo-Polynesian *taŋis 'weep', *Rasut < *Ratus 'hundred'.
Vocabulary
Lexical innovations only found in Northern Luzon languages include: *dutdut 'feather, body hair', *kəməl 'squeeze', *lətəg 'swell', *yəgyəg 'earthquake', *takdəg 'stand', *ʔubət 'buttocks'. Semantic shifts are observed e.g. in *ʔatəd 'give' (cf. Proto-Philippine *hatəd 'escort') and *laman 'wild pig' (cf. Proto-Philippine *laman 'flesh').[2]
^ abRobinson, Laura C. and Jason William Lobel (2013). "The Northeastern Luzon Subgroup of Philippine Languages." Oceanic Linguistics 52.1 (2013): 125-168.
^Himes, Ronald S. 2005. The Meso-Cordilleran Group of Philippine Languages. In Hsiu-chuan Liao and Carl R. Galvez Rubino (eds.), Current Issues in Philippine Linguistics and Anthropology: Parangal kay Lawrence A. Reid, 81-92. Manila, Philippines: Linguistic Society of the Philippines and SIL Philippines.
^Reid, Lawrence A. (2006). "On reconstructing the morphosyntax of Proto-Northern Luzon". Philippine Journal of Linguistics. 32 (1): 1–63.