A horos in the ancient Greek world marked boundaries between spaces: between public and private spaces, for example, or between properties. The marker on the right marks the boundary of sanctuary of the Tritopatores (the ancestors, worshipped in cult) in the ancient Kerameikos in Athens. On the left, inscribed with the words horos eimi tes agoras – "I am the boundary marker of the agora" – we have a 6th century horos from the Athenian agora, the central meeting and marketplace of the ancient city.
The reason horoi are important for our history of the classical world, however, is because of something that happened at the beginning, not at the end, of the 6th century BCE: the reforms of Solon. Athenian society of the 7th century BCE had become sharply divided between land-owning aristocrats and debt-crippled labourers, leading to an economic and social crisis. Solon's reforms centred around the seisachtheia – the "shaking off of the burdens" – which, as he himself writes in his poetry, involved moving the horoi from the land, which had been planted far and wide; "the land, once enslaved," he writes, "is now free."
So what did Solon mean by removing the boundary markers? Most historians believe that it refers to a radical cancellation of debts: land was often pledged as security for a debt, with markers like these horoi used to stake out the land owed; they would then be removed upon the cancellation of the obligation. These stones, then, aren't just markers of a physical divide in space: they are also reminders of the radical social reforms which swept Athens in the early 6th century BCE, presaging the advent of an entirely new kind of political system – democracy.