Synchromesh is a function of a transmission. The essence of synchromesh is to match the speed of an intended gear change to the speed of the current selected gear. By matching the speeds of the two gears, the gear change will be smooth and void of any grinding or noise. Prior to the advent of the synchromesh transmission, gear changes were very tricky and many people simply could not drive a stick-shift vehicle. The gears in a synchromesh-type transmission are always in motion and are activated or locked onto the shaft by a "dog collar."
In this type of transmission, the gears are always in motion when the vehicle is moving. In order to change gears, the synchromesh transmission uses linkages to slide the "dog collar" devices back and forth from one gear to another. When the dog collar meshes with a gear, it locks that gear to the transmission shaft, and the vehicle is then in that selected gear. As the dog collar leaves one gear, a special cone attached to the gear contacts the next gear, bringing it up to or down to the speed of the first gear. This allows the dog collar to slide easily into the next gear, thus meshing the gear and shaft together.
In early non-synchromesh transmissions, the gears were all separate and were best known as sliding mesh transmissions. In order to change gears in a non-synchromesh transmission, the shifting lever actually pulled one gear off of the drive shaft and slid or shifted another gear onto the shaft. This is where the term "shifting gears" comes from. The process was difficult at best, as the speed of the engine was required to match the speed of the desired gear and the operator used the gas pedal as much as the shifter when changing gears.
There is no need to double clutch a vehicle equipped with a synchromesh transmission. The act of double clutching was intended to bring the speed of the selected gear up to the same speed as the previous gear, thereby making the gear change effortless. The synchronized transmission does this for the driver by forcing the brass collar into the gear and allowing the dog collar to engage. The difference in the two styles of transmissions can best be compared by driving both the family vehicle and a riding mower. The auto can be shifted easily on the move, while the mower must stop to change gears.