14 C.F.R. § 221.41 – Routing.

(a) Required routing. The route or routes over which each fare applies shall be stated in the tariff in such manner that the following information can be definitely ascertained from the tariff:
1. (1) The carrier or carriers performing the transportation,
2. (2) The point or points of interchange between carriers if the route is a joint route (via two or more carriers),
3. (3) The intermediate points served on the carrier's or carriers' routes applicable between the origin and destination of the fare and the order in which such intermediate points are served.
(b) Individually stated routings—Method of publication. The routing required by paragraph (a) of this section shall be shown directly in connection with each fare or charge for transportation, or in a routing portion of the tariff (following the fare portion of the tariff), or in a governing routing tariff. When shown in the routing portion of the tariff or in a governing routing tariff, the fare from each point of origin to each point of destination shall bear a routing number and the corresponding routing numbers with their respective explanations of the applicable routings shall be arranged in numerical order in the routing portion of the tariff or in the governing routing tariff.