Spur gears are usually connected by placing them side by side on the same plane, with their teeth interlocking. In some cases, spur gears are positioned perpendicular to each other.
The movement is applied mainly on the first wheel or main gear. If the gears are placed side by side, the second wheel will move towards the opposite direction of the main gear's movement. The movement can either be clockwise or counterclockwise. Both gears will continue to move until a halt is placed on either of the gears.
Gear size does not necessarily have to be uniform. Even a large gear can be matched with a small gear. The only implication is that the large gear will have a much slower movement, translating to the small gear having faster movement.
Spur gears are not necessarily used in pairs, unlike bevel gears. There can be 3 or more spur gears used all at the same time, because using more spur gears allow for large gear reductions.
Ideally, the number of teeth of a gear should not be less than 12. The threshold is 8 teeth; gears with less than 8 will not engage well. For regular spur gears, ratios should never be anything larger than 10 to 1.