Here are some note I took while watching so vids. These are more for refrence and also just In case I forget (as math just goes over my head)
The size of the gear can be related back to the number of teeth that the gear has. Two gears connected to one another must have retrospective teeth to match one another which can be worked out with the ratio of ?/1. For example 2/1, 3/1 and so on.
For example using, but not limited to, the 12 X table you can work out the size of any gear to match.
(1x12) Small gear with 12 teeth. 1/1 ratio
(2x12) Mid gear with 24 teeth which is 2/1 ratio to the small gear. (2 times as big as small)
(3x12) Big gear with 36 teeth which is 1.5/1 ration to the mid gear but 3/1 to the smaller gear. (1.5 times as big as the mid)
This can be related to any even number for example 10. 1/1 ratio is 10, 2/1 would be 20 and then 30 would be 3/1. So as long as it keeps to the ratio, the gears can connect.