Hairpin

A hairpin is a blunt needle in the form of a narrow U to fixate hair into position.

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A road in the mountains often has hairpin turns, almost 180 degrees turns (with a shape like a hairpin) for zigzagging up and down slopes, to reduce the steepness. Although when hiking the steepness can be higher, it is also applied in trails. Similarly ramps for wheelchairs, buggies and shopping carts may zigzag.

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In genetics, a hairpin is a part of single-stranded DNA, usually a palindromic sequence, that aligns with itself to form a needle-like structure. A single-stranded DNA like

---CCTGCXXXXXXXGCAGG---

becomes

---C G---
   C G
   T A
   G C
   C G
   X X
  X   X
   X X
    X

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In music (specifically, sheet music), hairpins are symbols that indicate gradual changes in volume. They look like extended < or > symbols, kind of like hair pins, hence the name. See dynamics (music) for more information.