This type of gearbox is similar to the constant mesh type in that all the gears on the main shaft are in constant mesh with the corresponding gears on the layshaft. 

The gear on the layshaft are fixed to it while those on the main shaft are free to rotate on the same. 

Its working is also similar to the constant mesh type, but in the former there is one definite improvement over the latter. 

This is the provision of synchromesh device which avoids the necessity of double declutching. 

The parts which ultimately are to be engaged, are first brought into frictional contact which equalizes their speed, after which this may be engaged smoothly.

Figure show the construction and working of a synchromesh gearbox. 

In most of the earlier cars, however, the synchromesh devices were not fitted to all the gears as in shown in figure. 

They were fitted only on the high gears and on the low and reverse gears ordinary dog clutches are only provided. 

This was done to reduce the cost.