San Juan is the largest city in Puerto Rico by population. From 1899 to 1950 the municipality of San Juan excluded the township of Río Piedras. For this reason, population data and land area for the period make reference only to the Antiguo San Juan and Santurce ''barrios'', or subdivisions, of San Juan. The old municipality of Río Piedras constituted the third most populated city of Puerto Rico at the time of its annexation in 1951. Its strategic location south of the capital served as a junction for all the principal ways of transportation of the island and as a geographical entry to San Juan, which are factors that prompted Río Piedras's dramatic urban development in the 20th century.