The wider Lower Congo region in Central Africa is home to the Kikongo language (KLC), a vast language group of regional language varieties in four neighboring countries: Angola including Cabinda, the Democratic Republic of the Congo, the Republic of the Congo and Gabon. My doctoral research aimed to gain a better understanding of how the Kikongo Language Cluster (KLC), with a special focus on Cabinda, was formed over time. I have done so by adopting a historical-comparative linguistic approach to phonological, morpho-phonological and morphological variation that contemporary language varieties exhibit among themselves, and with respect to historical varieties for which written documentation is available. The oldest documents already date from almost four centuries ago. This research complements earlier studies on the history of the KLC within the BantUGent research group, in particular the lexicon-based language-genetic classifications. My doctoral dissertation is cumulative and includes four articles related to Bantu breathing and vowel number reduction from 7 to 5, progressive vowel harmony involving verb derivative suffixes, vowel harmony as part of the wider morpho-phonology of the verb ending *ide used for the recent past, and diminutives. I propose to talk about the second and fourth topic, explain about the original language maps of Cabinda that I made and about other data obtained during several consecutive on-site research visits. The conclusions present answers to the four main research questions of the dissertation, three pertaining to the historical development of the KLC and one to the planned promotion of the standardized Ibinda language in Cabinda.