An emergency bivvy — a full-length sleeved shelter in heat-reflective material, packed into a stuff sack about the size of a soda can.

A blanket you have to hold closed. A bivvy you climb into, so it stays shut while you sleep, shiver or pass the hours. That difference is the whole product.

The lining reflects your own body heat back at you rather than generating any, so it works best with a jacket on and something between you and the ground.

Bright orange, which is deliberate: you are easier to spot from the air or a ridgeline in an orange bag than in a grey one.

This lives in a day pack permanently. It weighs almost nothing and the one day it comes out is the day somebody stays out overnight who did not plan to.