A soldier's most important assets after his rifle are his feet. Leggings helped to keep debris out of the boots. Look for ones that have eight hooks. The ones with six are Marine leggings. $10 - 20 at militaria shows.
When you wear them the laces should be on the outside of the leg with the laces forward and the hooks to the rear.
This photo gives you a basic idea of how to put the leggings on. The eyelets should be to the outside, the strap under the boot, and lace them up tight. Most leggings are labeled with the size and whether they're left or right. There are four sizes (original) 1, 2, 3, & 4, and an additional size 5 in reproduction for those with large calves.
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