Here's what other sites are saying:
The Trangia Spirit Burner is the most versatile and user friendly alcohol burner on the market. Besides having a track record of reliable service that spans decades, it also has a couple of uniquely useful features. The simmer ring allows the burner to adjust from full to simmer and extinguishes the flame when closed completely. The twist on cap with o-ring seals the burner so you don't have to empty unused fuel between uses and prevents evaporation - less fuel wasted means less fuel you have to carry. Includes burner, simmer ring, and cap w/ o-ring.
- Mountains Plus
Trangia O-Rings seal the burner so you don't have to empty unused fuel between uses and it prevents evaporation! The less fuel that gets wasted means the less fuel you will have to carry. Let your burner cool before replacing the cap to avoid melting the o-ring, but just in case, this 2 pack has you covered.