"The Main Line" describes the western suburbs of Philadelphia along Lancaster Avenue (US Rt 30) and the former Pennsylvania Railroad mail line, extending from Philadelphia's city limits to approximately Malvern, and encompassing approximately 200 square miles. The principal municipalities comprising the Main Line are (from east to west) Lower Merion Township, Haverford Township, Radnor Township, Tredyffrin Township, Easttown Township, Malvern Borough, and Willistown Township.

Farmland has become almost entirely residential in the last hundred years. A multitude of hills and valleys dictates the interestingly irregular roads and unique locations of many homes. In accordance with long-term planning, current industry and commerce exists only in a very few original locations: close to Lancaster Avenue, at the center of the individual towns, or in Bala-Cynwyd along City Avenue where huge apartment, office, and retail buildings have taken root. Apartments are largely found along Montgomery and Lancaster avenues and near train stations.

Many median-income families live close to transportation in the sub-divisions that emerged in the twentieth century in Penn Wynne, Merion, Bala-Cynwyd, and Wynnewood. Houses still remain large and costly in many areas, though estates are constantly being subdivided. The Main Line is known for its multimillion-dollar homes, exclusivity, and upscale shopping. The Main Line is also home to some of the best public and private schools in the country.