Single planetary gear sets operate under certain laws, five in all, to achieve various drives on vehicles. Let's review the five laws that govern these drives. First there's neutral, governed by law number one. When there is an input and an output, but no reactionary, the result is neutral. Next is law number two. When the planet carrier is the output and there is a reactionary, we get gear reduction. This sets our vehicle in motion. After this comes law number three. When the planet carrier is the input, the result is overdrive. Then there's law number four. When the planet carrier is the reactionary, we get reverse. And finally, law number five. When any two members are locked together, the result is direct drive. And for the mobility required today, many vehicles must have more speeds and capabilities at their disposal. One way to get this required mobility is by changing the relative sizes of gear members in a set. To widen the selection of ratios still more, we can use two or more sets in combination. Such combinations called multiple planetary gear sets are typical of certain transmissions. To demonstrate how connecting the sets multiplies ratios, let's use just two sets first. We'll put both sets in reduction. One set has a ratio of three to one and the other a ratio of four to one. Therefore we get a total reduction of twelve to one. This is the lowest speed. Now by putting the small gear set in direct drive and keeping the large one in reduction, we get a total reduction of four to one. This is second speed. By putting the small set in reduction and the large one in direct drive, we get a three to one reduction or third speed. With both sets of gears placed in direct drive, we get a ratio of one to one with no gear reduction. That's fine for forward, but vehicles have to back up too. How do we get reverse? Well, to our two sets we could hook off a simple reverse set. With the planet carrier made reactionary by a band, this would give us reverse. We could also transmit forward through the simple reverse set by putting it in direct drive. So a reverse would be too bulky. Space and weight can be saved by using what is called a compound reverse, which you will come across in certain truck transmissions. With our compound reverse model, let's keep in mind that this is our front set. This is the intermediate set and this is the reverse unit. We'll see how this works. Let's build up the compound reverse unit step by step. The ring gear of the intermediate set is connected by this shaft to the sun gear of the reverse unit. This connecting shaft is hollowed. We'll allow the output shaft to pass through it and connect the planet carrier of the intermediate set to the planet carrier of the reverse unit. We have the planet carriers of these two units connected by the output shaft. And we have the ring gear of the intermediate set connected to the sun gear of the reverse unit. The next thing we add to the reverse unit is its ring gear, which can be held reactionary by a cone type clutch. Now let's follow the flow of power in this compound reverse set up. The sun gear is the input in the intermediate set. The load on the output shaft, which connects to the intermediate planet carrier, makes it the reactionary. With this planet carrier reactionary, the intermediate ring gear will move in a direction opposite to that of the sun gear, which is driving. This gives us the reverse movement we want. This reverse movement of the intermediate ring gear is transmitted by this connecting shaft to the sun gear of the reverse unit. By making the ring gear of the reverse unit reactionary, the planet carrier becomes the output and follows the direction of the sun gear, which is driving. The reverse planet carrier transmits the reverse through the output shaft to the wheels.