To ward off the threat of intruders, the noblemen of Edo-era Japan devised a clever way to alert them to the presence of enemies.

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Throughout the centuries in Japan, highly skilled craftsmen have made a stunning mark on its landscape. Exquisite castles, once the homes of Japanese daimyō, speckle the countryside. And yet an unusual sound echoes around their elaborate hallways and corridors. A kind of birdsong chimes from the movement of the floorboards with every footstep. But there’s a clever purpose behind the chirping.

From 1603 to 1868, Japan was ruled by the Tokugawa shogunate; the era was known as the Edo period. It was an era of intricate civil arrangements, in which everyone knew their social status. Emperors ruled the land, yet they wielded little actual power themselves. They, however, were supported by a cast of shōguns, daimyō and lords, who numbered in their hundreds.

These noblemen’s prestige were affirmed by grandiose rituals, as well as elaborate residences. However, the flooring in their palatial homes may not at first seem fitting of such extravagant architecture. Every footstep through the distinguished hallways created a cacophony of creaks and screeches through the corridor. But far from being the result of inferior workmanship, the noise was in fact cleverly orchestrated.

The sound created by the friction in the floor’s structure is called uguisubari; literally, “bush warbler guard watch,” or more commonly “nightingale floors.” When floorboards were installed, some movement was built into their clamps. A double spike rubs against its respective receptor, the friction creating a chime redolent of a nightingale’s call. The purpose was to alert residents to intruders.

So how would noblemen distinguish the everyday movement of guards and palace staff from that of unwanted guests? Well, staff were trained to move across the floors following an ascribed pattern. Therefore, any deviation from a recognized rhythm would be likely to signify that an intruder was present, raising the alarm. Nightingale floors are preserved today in the Ninomaru Palace and at Chionin Temple, in its Daihojo Hall.