keystone

Everyday

1. A keystone is the central wedge-shaped stone at the top of an arch that holds all the other stones in place. tree →
2. A keystone is the most important part of a plan, idea or system, which everything else depends on. tree →
3. (science) A keystone species is an animal or plant that its whole ecosystem depends on, like sea otters or beavers. tree →

In the lessons

In history and design lessons, Roman bridges and church arches stand because of the keystone. In science lessons about food webs, removing a keystone species makes the ecosystem change greatly. Pennsylvania is called «the Keystone State». Phrases: «keystone species», «the keystone of».

Examples

• The keystone at the top holds the whole arch together. tree →
• When the builders put in the keystone, they could remove the wooden frame. tree →
• Sea otters are a keystone species because they eat sea urchins that destroy kelp forests. tree →
• Reading is the keystone of learning. tree →
• Beavers are a keystone species that build dams and make ponds. tree →
• Pennsylvania is known as the Keystone State. tree →

Note

Countable. In an arch, the keystone is pushed from both sides, which locks it in; this is why arches carry heavy loads. «Keystone» is not the same as «cornerstone» (a stone at the corner of a building), though both can mean «the most important part».