This invention relates generally to equipment for vehicles, such as pickup trucks, and more particularly to a releasably attachable and adjustable rack system having sliding connections and many other accessories for attaching to the overhead rack of the vehicle. 
Vehicle racks and systems are very well known in the art.  They can connect to various parts of the vehicle such as a cab and side walls of a truck bed.  The rack systems that typically connect to a truck bed include a frame-like structure that includes a connection for attaching the rack to a vehicle as well as a structure for receiving accessories for customizing the rack system.  For example, the rack system commonly includes a base or footing, a vertical tube or post, a cross-rail and a structure for connecting these components together. 
For example, FIGS.  1-4 Illustrate a known prior art rack system 10.  FIGS.  1-4 are of interest because they employ components that are permanently welded together and use gusseted braces 12 to form the desired configuration.  As in FIG.  1, the base 14 is welded to the bottom of an upright member 16, which is further reinforced by the gusseted brace 12.  The top of the tube is also welded to a top saddle member 18, which is also further reinforced by a gusseted brace 12.  As a result, a unitary rack member, that includes a base 14, an upright tube 16 and a saddle 18, is provided.  This general construction can be seen in U.S. Pat.  No.  7,014,236.  This patent is of interest for its teachings of a vehicle rack system 10 that uses gusseted braces 12 to better secure rack members relative to each other and to provide a more rigid joint connection. 
FIG.  2 is a close-up view of a prior art rack system 10 that includes a horizontal base member 14 that is clamped to the side wall 20 of a vehicle, such as a pick up truck.  A vertical member 16 is permanently welded to the horizontal member 14 to provide an upright structure.  A top saddle 18 is a also permanently attached to the top of the vertical member 16 for receipt of a top rail 22 and other accessories thereon, as is well known in the art.  Such a known construction can be seen in FIG.  3 where a top rail 22 can support equipment, such as ladders, and the like.  The clamps 24 are adjustable to permit the horizontal member 14, and thereby the vertical member 16, to permit a customized installation, as can be seen in FIG.  4.  Typically, a set of four horizontal members 14 and associated vertical members 16 are used to provide a pair of top rails 22 for location at a desired distance D from each other, as can be seen in FIG.  4.  This construction is so well known in the art that further discussion herein is not required. 
Although the prior art systems function acceptably as rack systems, they suffer from a number of disadvantages that make them undesirable.  For example, a base horizontal member 14, a vertical upright member 16 and top saddle 18 are typically welded together, which results in a structure that is unacceptably large in size, which makes shipping very difficult because a large box must be used.  This oversize packaging adds costs to shipping and, as a result, adds to the cost of the rack system.  Also, such a prior art structure is very labor intensive because parts must be welded together.  Such welding requires expensive parts and components and is a time consuming and expensive manufacturing process.  Missed weld locations will also make the rack “non functional” out of the box forcing customer to return the product.  Moreover, welded products add weight to the overall package thereby adding cost and, unfortunately, welds, best seen as 26 in FIG.  4, are susceptible to failure over time.  If there is a failure, it is very difficult and expensive to replace only a portion of the entire welded structure.  So, as a result, wasteful replacement of the entire welded unitary structure is typically required. 
Also, a structure that is welded requires cumbersome gusseted braces 12 to supplement the welds so that acceptable rigidity can be achieved.  These braces 12 add cost and further increase manufacturing time thereby further adding to the overall costs of production. 
A permanently welded structure takes away most if not all of the customization options because the horizontal member, the vertical member and saddle are permanently fixed to each other in a given configuration.  Thus, such a permanently welded structure makes it more difficult to provide options and flexibility for the user.  As a result, prior art rack systems 10 are not conducive to a desirable modular rack system. 
Finally, the permanent welds 26 and cross-braces 12 and gussets 12 of a rack system 10 are very unattractive in appearance as these welds 26 and braces 12 are readily visible. 
In view of the foregoing, there is a demand for a rack system that eliminates welds to increase manufacturing capacity by removing the tedious welding process.  There is a demand to directly cast in the reinforcing geometry into the base and the saddle to obviate the need for gusseted braces.  There is a demand for a rack system that is even stiffer without gusseted braces by using a component that is thicker in the existing welded locations from the prior art rack system 10.  There is also a need for a system that is modular so the user can customize and configure the system to what they need by mixing and matching horizontal bases, vertical upright members and top saddles, as well as top rails and accessories.  A new rack system is desired that does not use a unitary welded structure to enable different variations of the horizontal member, vertical member and top saddle to be used and to obviate the need for expensive welding.  Also, there is a need for a modular system so if one part of the system fails, only one small component is replaced rather than a larger unitary welded part.  There is a also a need for a rack system that is more attractive and sleek in appearance and one that looks like a unitary structure but is actually a modular bolted component system.