장
jang · TOPIK 1
sheet (counter for flat objects)
📄 張 = to stretch out; a sheet is something stretched flat, so count it in jang
The hanja 張 contains 弓 (bow) and originally meant to draw a bow taut, then to stretch or spread. Anything spread out flat, like paper, tickets or photos, is counted with 장: 종이 한 장 = one sheet of paper.