And she said, According unto your words, so be it. And she sent them away, and they departed: and she bound the scarlet line in the window. (Joshua 2:21)

Charles Spurgeon writes:

Rahab was told to tie the scarlet thread in the window, and she did it; there was exact obedience. It was not merely a thread, a line, but the scarlet line. She did not sunstitute a blue, or a green, or a white line. The order was this scarlet line, not another, and she took that particular one.

You can read the rest of this devotion at Christian Research Net.