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In the field, especially when packing light, a good cup is more than just a receptacle for beverages and soup. It's a dinner plate and a cook pot, a wash basin and even a trowel.
Each design has advantages, of course, as well as disadvantages. I've probably used various Sierra cups more than any other type, even though their relatively small capacity almost always disappoints.
Lately, with an eye on condensing my kit as much as is practical, I'm leaning more toward a "nesting" setup -- thus the GI canteen cup and also the stainless-steel GSI, which mates with a common Nalgene bottle in similar fashion. A cup's volume has become more important to me, too, primarily for water-purification purposes, so it's no accident that the GI and GSI are the largest of the bunch.
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We cemented our bond in 1972, I think, at Philmont Scout Ranch. Over 75 miles of dusty trail it hung from a pack strap or my belt, clink-clanking at the end of a rawhide thong. It served me hot cocoa and tepid bug juice, beef stew and mac'n'cheese. A half-dozen years later, it drew icy water from Kintla Creek.
Other cups may be bigger or more versatile, but that one's a keeper.