builder's frame for measuring
栝 centers on a wooden tool used for measuring: a builder's frame or carpenter's square. The meaning is narrow and technical, tied to construction.
栝 combines 木 (tree/wood) with 舌, which likely contributes the sound. The character originally referred to a wooden measuring tool, consistent with the wood radical.
The left side is wood 木, and the right side looks like a tongue 舌 sticking out. Imagine a wooden frame with a tongue-like sliding part used for measuring in carpentry.
For テン, picture a carpenter using the wooden measuring frame under a tent: tent -> テン, and the frame helps measure the tent poles.