The mountain system is located in central Mexico. "Western" in Spanish is "occidental," meaning Sierra Madre Occidental is to the west. "Eastern" in Spanish is "oriental," meaning Sierra Madre Oriental is to the east. "Southern" in Spanish is "del sur," meaning Sierra Madre del Sur is to the south.
Fun Fact: Sierra Madre Oriental contains the largest point in Mexico called the Pico de Orizaba (or "Peak of Orizaba [Mexican city]").