How do I link an object together?
Objects in the Second Life virtual world are built out of basic shapes, called primitives. You can link several primitives (prims) together to create one cohesive object. A linked object is, for all intents and purposes, considered one object. It has one name, acts as one object (for example, if physics are enabled on it), and it cannot be broken apart unless you Unlink it yourself. However, a linked object still counts as the sum of its prims when determining your land's object limits. One prim of the object is considered the parent or root link. The name of the parent link is the name of the whole linked object. The inventory of the root prim is, for most purposes, the inventory of the whole object. The center (or origin) of the root prim is the center of the whole object, even if the root prim is not the physical center of the object itself. Vehicle scripts look at the root prim's orientation to determine the "front" of the vehicle. As a result, it is important which prim you select as the root prim.
Linking an object
1) If you are not in the object Editor already, right-click any object and choose Edit, or open the Editor with Ctrl-3. 2) Click on any empty area to make sure you do not have any objects selected. 3) Hold down the Shift key, and click on each prim you want to link together. The last prim you select will be the root prim. 4) Once you have all the prims selected, open the Tools menu and select Link, or press Ctrl-L.
A linked object cannot exceed 30 meters in any dimension. Vehicles, or any physics-enabled object, cannot have more than 31 prims. Note that any rider sitting on the vehicle will be considered a linked prim on that vehicle, so the vehicle itself should remain under 30 prims. (A vehicle can turn on its physics, then have multiple riders sit on it). There is no nesting of linked groups. In other words, if you link a third object to two objects already linked and then unlink them it will not yield two groups. Instead it will yield three.
Additional information is available on the Linkability Rules wiki page.