Best emergency shelterFor the day someone cannot walk further
SOL Emergency Bivvy
A hundred grams that keeps an injured or exhausted person alive while help walks down for them — you will almost certainly never open it, which is exactly the point.

Fox 40 Pealess Whistle
Pealess, ~120 dB, ~10 g
Carries across a valley when a voice does not and works when you are too cold or too winded to shout — pealess means it still works soaked or frozen, which the corked kind does not.
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Do not buy this if: Nobody. Clip it to the chest strap, not inside the lid.
Everything else we recommend in emergency & comms, including the ones we think suit a different trekker.
Best emergency shelterSOL Emergency Bivvy
A hundred grams that keeps an injured or exhausted person alive while help walks down for them — you will almost certainly never open it, which is exactly the point.
Most used item nobody listsTape and a small repair kit
A metre of tape wrapped around a trekking pole has salvaged more trekking days than any other single item we carry — it will not fix a delaminated sole permanently, but it will get you to the next village.