The African American Heritage Trail winds across the District, identifying sites that are important in local and national history and culture. The trail consists of a searchable database, a free booklet and sites marked by historical plaques. At 10 a.m. Tuesday, the trail’s 100th plaque will be unveiled at 931 R St. NW, the former location of Fire Engine Company No. 4 — the city’s first all-black fire station. The booklet on the trail can be downloaded online or picked up at a neighborhood location. For more information, go to culturaltourismdc.org.