MONTGOMERY, Ala. — From a certain spot here on Dexter Avenue, a person can see the place where Jefferson Davis was inaugurated as president of the Confederacy, the church where a young Martin Luther King Jr. rose to prominence and the corner where Rosa Parks got on the bus.

All of this is marked, as it seems nearly every corner of downtown Montgomery is marked. There are scores of monuments, memorials and historical signs noting sites relating to the Confederacy or to the civil rights movement, from the Confederate postal headquarters to the homes of lesser-known figures in the 1955 bus boycott.

But there is only one sign, added in 2001 a few yards away from Rosa Parks’s bus stop, explaining the city’s role in the underlying cause for so many of the others: “Montgomery’s Slave Markets,” it reads, followed by a paragraph of explanation.

On Tuesday, the Equal Justice Initiative, an organization that since 1989 has provided legal representation to poor defendants and prisoners, will be unveiling three new markers describing in greater detail the city’s role in the domestic slave trade. That role was substantial: in 1860, there were more than 20,000 slaves in Montgomery, a larger number than in New Orleans or Natchez, Miss., and Dexter Avenue was a busy corridor of slave pens and depots....