The relations between two countries is measured on a scale from -200 (worse) to +200 (best). Relations can be raised by acts such as sending gifts or accepting royal marriages, but can also be lowered by sending insults or declaring war. Relations dictate the likelihood that a nation will accept your diplomatic proposals. In the case of Personal Unions relations must be kept above 0 for the union to continue. Offers of Vassalisation and Diplomatic annexation require relations above 190. Having relations above 100 with the Papacy prevents Excommunication.