Tooth size, style and pitch must be the same. And if the gears are helical they must have the same twist, in opposite directions.
Matching the gears to mesh properly.
meshing gears
are when all gears are constantly engaged
Sliding mesh - This is where the gears are not in constant mesh with each other and the gears on the main shaft slide over to mesh with the gears on the lay shaft/counter shaft. Synchromesh - the problem with constant mesh is that the gears crash into each other when changing gear because there travelling at different rpms (one at engine speed and the other a lot slower) They dont line up properly when spinning and when they go to mesh they end up grinding. So the awnser is Synchromesh which uses Baulk rings to lock onto the gear first before the sliding sleeve comes over to lock on. Difficult to explain how it works but basically its a smooth gear change.
Don't really know what you're tinking of with "mesh together"... On a bike with external gears the gears will not mesh. There's the chain acting as a go-between to transfer the energy. On an internally geared bike you will have cogwheels that are interlocking and driving each other.
what is the pitch point
Crown and pinion gears are two types of mechanical gears that mesh together.
where the gears are always engaged.
a crown and pinion is a st of gears that mesh at an angle.
Usually a single part made up of a serious of Gears that mesh with various gears of main shaft
The gears of the starter mesh with the gears on the engine flywheel which is located somewhere where the engine connects to the tranny
When the "bendix gear" is engaged by the starter, sometimes if all is not aligned properly, the gears will not mesh causing gear tooth breakage. also excessive heat can cause warpage.