One elastic bungee strap that turns a rolling case into a two-bag trolley. It loops over the telescopic handle and tightens around a duffel, a tote or a folded coat, holding the load steady instead of letting it slide off at every kerb.
The length adjusts, so it grips a soft weekender as readily as a hard-sided cabin case, and it packs down to nothing when you are not using it - roll it up and drop it in a side pocket.
Sold as a single strap. It earns its keep on airport transfers, on train changes with a platform to cross, and on the walk from the car park where you would otherwise be carrying a bag in each hand.
