1. Anti-Jemmy Devices / Strips (for Doors and sometimes Windows): While sometimes simply called “anti-blocks” in a broader security context, these are designed to prevent prying attacks.

   • Purpose: These are robust security additions, often steel strips or claws, fitted to the edges of uPVC doors (and occasionally windows) to resist attempts to force them open with a jemmy bar or similar tool.

   • How they work: They cover the vulnerable gap between the door/window sash and the frame, making it extremely difficult for an intruder to insert a tool and apply leverage to force the lock or hinges.

   • Types: Can be “anti-jemmy strips” that run along the full length of the lock side, or “hinge protectors” (often called “dog bolts” or “security bolts”) that reinforce the hinge side. Some “anti-jemmy devices” are small, specific blocks for windows, like the “CAL Tiger Anti-Jemmy Device” for sliding systems mentioned in search results, which specifically grips the frame tighter when force is applied.

Important Note on “Anti-Block” in Manufacturing: It’s worth noting that in the manufacturing of plastic films and sheets, “anti-block agents” are additives used in the polymer to prevent layers of plastic from sticking together. This is a very different context and not what you’d typically install on a finished uPVC window or door. Based on your previous questions about uPVC components, you are almost certainly referring to the security features.