Seatbelts lock to keep passengers from flying around in an accident. Under normal, non-emergency conditions we're used to today's seatbelts moving Seatbelts with an Automatic Locking Retractor (which isn't very common these days) lock whether you want them to or not; think about the old-school lap belt that keeps getting tighter and tighter every time you move. It has just one mode: locked. Seatbelts with an Emergency Locking Retractor only lock in an emergency and can't be used on their own to install a car seat, as the car seat won't stay in place properly while you drive around. These are common in many countries. It has two modes: unlocked for most situations, and locked when you stop short or collide with something. Most modern seatbelts in the US and some other countries have a switchable retractor. That means it can be unlocked most of the time and locked in an emergency or you can switch it to locked mode to install a car seat using the instructions below.