When senior journalist Edgar Sandoval had a landmine planted in his back yard after moving to Florida, he didn’t know that his dog “Gracias” — hence the name of his memoir — would accompany him through his cancer treatment and, later, during his battle with death. On Tuesday, the Humane Society of the United States officially listed the majority of major airlines as “best friend” airlines in 2015, based on its Social Pet Index, a ranking of which carriers offer pet carriers in economy class. This is the first time that all major airlines have been included. (Delta Air Lines and Southwest Airlines were excluded, for example, because they have 100 percent animal per-passenger service.) The index was devised by matching social-media posts from the past three years and comes with a few “bare bones” recommendations. If you’re flying with your dog, cat, parrot, iguana, budgie, shark or guinea pig, the Humane Society suggests wearing your human outfit over your pet carrier (they can get cold) and “dress up your pet with T-shirts, swimsuits or hats.” If you’re flying on Southwest, which favors a cartoon-animal-wing motif, it recommends strapping your pet to your ankles or chaining him to the seat. The scale used in the index is based on aircraft weight to accommodate the weight of large pets (which also should be stowed under seats at all times). That way, your (mostly empty) jetliner doesn’t have to lower the height of the ceiling to give your animal a better view of the cockpit. That increase in restraint height is a preferred design for cargo airlines, who have to allow for smaller cargo containers on planes.