I Wonder Why...?

This small stone slab in Madrid’s city square marks what is popularly considered the geographical center of Spain, although it isn't the 'exact' centre, it is not far off. The marker sets the Zero Kilometer point, the point from which distances are measured in Spain—specifically, it’s the starting point for measuring the distances of the country’s six national roads, A-1 to A-6.

Kilometre Zero is also the starting point for Madrid’s numbered streets; the closer to the Km 0 mark, the lower the street number.

These traditional markers are common to find in many countries, often located in the capital city. The most famous milestone marker however is the “Golden Milestone” (Milliarium Aureum) in Ancient Rome, which is believed to be the source of the saying “all roads lead to Rome.”
Spain’s Kilometre Zero is in the middle of Madrid’s Puerta del Sol, the historic semi-circular city square. The slab is quite easy to miss, situated on the ground outside the clock tower on the old Post Office building. 

The plaque was placed here in 1950 and replaced with a newer stone in 2002 and again in 2009. It was turned around 180 degrees by mistake in 2002 during a reform of the square. The plaque was renewed in 2009, during the roadworks of the Puerta del Sol square, and this time placed in the right position.