Squeaky floors are annoying, which is why when you search for “squeaky floors” in Google, you get hundreds of articles offering you tips and advice on how to make your floors squeak less. But in ancient Japanese societies, especially among royal families, squeaky floors were not hated but desired—because a floor that screeched and groaned with each step was an effective anti-burglar alarm system.

Some Japanese castles built during the Edo period had these kind of flooring in the hallways. They are known as uguisu-bari, where uguisu refers to the Japanese bush-warbler or the Japanese Nightingale—a very shy bird that prefers to stay hidden among the foliage, but its distinctive breeding call can be heard throughout much of Japan from the start of spring.

It is said that these specially designed creaking floors sound much like the bush warbler singing. In English, they are known as nightingale floors.

Inside Ninomaru Palace of the Nijo castle where nightingale floors are installed. Photo credit: Fran Sastre/Flickr

Nightingale floors were popular during the Edo period—a period stretching from the early 17th century to the mid-19th century, when Japan was under the rule of the Tokugawa shogunate. The Edo period was considerably peaceful with fewer wars and rivalry, but the threat from the shogun’s subordinate feudal lords and other enemies was always there.

When Tokugawa Ieyasu, the founder of the Tokugawa shogunate, built the Nijō Castle in Kyoto as his residence, he ordered his carpenters to install nightingale floors in its hallways so that any assassin trying to sneak into the castle will immediately alert his guards.

While dry boards naturally creak under pressure, nightingale floors have extra metal clamps located between the beams that support the floorboards of the corridor. Each clamp has two spike holes through which an iron spike goes through. When someone walks over the boards, the clamp moves up and down causing the spike to rub against the clamp, producing a shrilling noise.

Because it was impossible walk across these hallways without making their presence known, guards and sentries developed a special rhythm that they would use when walking over the boards so that other guards know that it’s one of them. If the guards heard the floors singing a different tune, they knew they had an uninvited guest, and that it was time to sound the alarm.

The two best places to see and experience nightingale floors are the Nijo castle in Kyoto, and Chion-in, a temple where the Tokugawa family used to stay.

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Ninomaru Palace. Photo credit: ERIC SALARD/Flickr