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The selection of door and window anti-fall chain length should be combined with the opening requirements of doors and windows and the safety distance:

- First, determine the safe opening angle of doors and windows (usually recommended not to exceed 30-45 degrees, which can ensure ventilation and avoid dangers caused by excessive opening).

- Measure the straight-line distance between the installation points when the door/window is opened to the safe angle. The length of the anti-fall chain should be slightly shorter than this distance (reserve 1-2 cm to avoid excessive force after being fully tightened).

- For small-sized doors and windows (such as width ≤ 80 cm), anti-fall chains with a length of 20-30 cm can be selected; for medium-sized doors and windows (width 80-120 cm), anti-fall chains with a length of 30-40 cm are suitable; for large-sized doors and windows (width > 120 cm), it is recommended to choose anti-fall chains with a length of 40-50 cm, but the load-bearing capacity of the doors and windows should also be considered to avoid excessive opening angle due to too long chains.

- For special scenarios (such as children's rooms), shorter anti-fall chains (20-25 cm) can be selected to further limit the opening range.