I'm selling this because I bought a pack of 4 but only used one! It's a great product for childproofing door locks. This product originally sells for around 3,700 yen for a set of 4. It's a very sturdy product. Product Details: 【Material】: High-quality aluminum alloy, durable and scratch-resistant. 【Anti-theft measures】: Auxiliary lock for window glass and aluminum frames. Prevents mischief by children and pets, and prevents crime. 【Ventilation】: You can lock the window frame while it's slightly open, which is convenient for ventilation in bathrooms, kitchens, and toilets. 【Lightweight design】: Twist to lock, making it convenient and fast, and there's no need to use other keys, making it difficult for children to open. 【How to use】: Insert the window lock into the groove and turn the knob to secure it firmly.