Why do I measure the length inside instead of outside?

为什么只量取内侧的长度而不是外侧的长度呢？

youdao

One four foot length of wooden dowel that fits inside the pipe.

一个四英尺长的木制桩相适应的管内。

youdao

One four foot length of wooden dowel that fits inside the pipe.

一个四英尺长的木制桩相适应的管内。

youdao