Requiring that two embeddings be isotopic is a stronger requirement than that they be homotopic. For example, the map from the interval [−1, 1] into the real numbers defined by "f"("x") = −"x" is "not" isotopic to the identity "g"("x") = "x". Any homotopy from "f" to the identity would have to exchange the endpoints, which would mean that they would have to 'pass through' each other. Moreover, "f" has changed the orientation of the interval and "g" has not, which is impossible under an isotopy. However, the maps are homotopic; one homotopy from "f" to the identity is "H": [−1, 1] × [0, 1] → [−1, 1] given by "H"("x", "y") = 2"yx" − "x".