[4] This highway consists of one lane in each direction for most of its length, except in the southern terminus, where Calle San Ramón is a one-way street with a single lane from Calle Colón to PR-891.

[6] Between Barrio Pueblo and Abras, PR-821 becomes rural, connecting several neighborhoods and sectors until its transition to PR-630 near the Vega Alta municipal limit.

[4] Prior to its numerical designation, PR-821 was only known as Camino del Abra.

The current numerical designation corresponds to the 1953 Puerto Rico highway renumbering, a process implemented by the Puerto Rico Department of Transportation and Public Works (Spanish: Departamento de Transportación y Obras Públicas) that increased the insular highway network to connect existing routes with different locations around Puerto Rico.

This Puerto Rico road-related article is a stub.