A synchronizer performs two important functions. Its main function is to bring components that are rotating at different speeds to a common or synchronized speed. In a transmission, a synchronizer ensures that the main shaft and the main shaft gear about to be locked to it are both rotating at the same speed. The second function of the synchronizer is to actually lock these components together. If the driver and the synchronizer are both doing their jobs, the result is a clash-free shift. Most standard transmissions having five to seven forward speeds are completely synchronized except for first and reverse gears. The reason for not synchronizing first and reverse gears is that they are only selected when the vehicle is stationary. Synchronizing of engine speed and driveline speed is not required when the drive shaft is not turning; in fact, a synchronizer could cause hard shifting in these gear positions because a synchronizer needs gear rotation to do its job.
Many transmissions use both main and auxiliary gearing to provide a large number of forward ratios. In these transmissions synchronizers are normally used only in the two- or three-speed auxiliary gearing. Block or cone and pin synchronizers are the most common in heavy-duty truck applications. Both block or cone and pin synchronizers are often referred to as sliding clutches.