The Senate is a chamber of territorial representation in which, regardless of population, every province assigns four members by direct election. The rest of the seats are distributed through indirect vote by the parliaments of the autonomous regions. Every elector is able to vote three of the candidates on a single ballot. The Senate holds less power than the Congress of Deputies and its power concerns the autonomous communities.
The current electoral system of Spain was established in 1985, updating a previous law of 1977, based on d’Hondt system of proportional representation. The Spanish electoral system allows the population of each one of the 50 provinces (plus Ceuta and Melilla) to have a minimum representation according to their size. The adopted system underlines the historic and political context in a country with special regional characteristics and great demographic imbalances. Beside the provinces, Spain is divided in 17 autonomous communities which exercise legislative powers through their own parliamentary assemblies, in compliance with the constitution of their regional basic laws.
The Spanish electoral system tends to overrepresent small provinces and simultaneously tends to favor major parties while smaller and regional parties tend to be underrepresented.
Sources: Spanish Ministry of Interior, Wikipedia.