US Patent 7770267 Buckle

Two wedges extended from the first arm and the second arm of the waist-strap fastener respectively are utilized to engage with the waist-strap button. With inclined surfaces that face with each other on the first arm and the second arm, the mounting section of the waist-strap button is capable of squeezing through the gap between the first arm and the second arm. With the surfaces on the other side of the wedges respectively, the mounting section of the waist-strap button is securely held by the surfaces such that the waist-strap button is secured in the waist-strap fastener.