Charles Ellicott Commentary Joshua 2:18

"Behold, when we come into the land, thou shalt bind this line of scarlet thread in the window which thou didst let us down by: and thou shalt gather unto thee into the house thy father, and thy mother, and thy brethren, and all thy father`s household." — Joshua 2:18 (ASV)

The window by which you let us down. —It seems almost needless to observe that the scarlet line and the cord by which the men were lowered are not the same thing, but are described by different words in the original. It would have been preposterous to require Rahab to display in her window the means by which the spies had escaped. It would at once have declared the tale to all beholders—the very thing Rahab was pledged not to do.

The "line of scarlet thread" and "stalks of flax" on the roof were probably parts of the same business. Consequently, there would be nothing unusual in what was exhibited at the window. It would, however, be a sufficient token to those who were in the secret to enable them to identify the house.