Two slim pealess rescue whistles — flat enough to sit under a pack strap or inside a wallet without bulk.

Flat is the useful part. A round whistle digs in and gets taken off; a slim one disappears against a strap and stays put, which is the only way a whistle is ever there when it is needed.

Pealess, so no cork ball to jam or freeze. Works soaked.

Three blasts, repeated, is the recognised distress signal. One blast means I am here.

Two per pack at nine dollars, so there is no reason for a second person in the group to go without.