There has been a fair amount of hallabaloo recently regarding the joinery best suited for the stile and rail garden gates developed by CPW in the early 90's.  So here we'll take some time to review a few of examples of this much-maligned technique. 

A quick word, in passing, to acknowledge the carpenter's Butt Joint.  This is most likely responsible for the vast majority of garden gates in existence today.  Fastened with nails or screws, we have an assembly that is also often accompanied by a wood, or cable, diagonal brace in a losing effort to forestall the certainy of a sagging assembly. 

The Half-Lap joint.  Originally, our gates utilized this joint on all gates, illustrated thoroughly in an article for Fine Homebuilding in the Spring of 1997. But in time we came to realize the limitations of the half-lap.
Advantages: 
1) A fairly simple joint to fabricate, providing a large mating area, or what might be called the 'glue-mass.'
Disadvantages
1)  Wood breathes with the seasons.  The red arrows indicates in what direction the wood will breath, expanding and contracting in wet and dry weather.  This breathing occurs perpendicular to the direction of the wood grain. The stile, which is the vertical stock of the gate, breathes left and right, or horizontally.  This is why there is a 3/8" clearance configured between the net width of you gate and the post or jamb.  Now . . .the rails, which are the horizontal stock between the stiles, is going to breath vertically, or perpendicular to the horizontal rail.  In direct contrast to the stile's movement. 
------In furniture design, this is a principle that cannot be slighted, as the result can often lead to a tug of war that results in one of the two members suffering the severe damage of checks and fissures.  Why?  Because one of the two members is trying desperately to simply breath, which is it's organic right, and yet it is being held in check by the half-lap of the corresponding member.  The result, again, is stress.  Stress is bad.  Stress is at the root of not only schizophrenia and paranoia and a whole bevy of nervous disorders, but it can also result in this half-lap joint opening, loosening, and the opposing member checking and cracking just as a seemingly sane soul begins to slip from sanity.
2)  The second issue that arises with the half-lap joint is how the top of the joint is essentially exposed to the rainfall.  This in itself is not the most favorable scenario in that a golden rule in exterior carpentry and woodwork is to avoid allowing end-grain to ever be exposed to the vertical fall of the weather, such as rain and snow and the heat of sunlight.
3)  The third issue is that the top of the exposed half-lap joint has nothing to thwart it's placement beyond the strength of the adhesives.  As we see in the far right illustration, above, is how once the joint opens from the exposure to direct weather, then checks and cracks from the opposing grain directions, the rail begins to crack and check under the stress and drops, or sags, with nothing to prevent the lure of  a beckoning gravity. 

We next turn to the through tenon.  Here we double the glue mass, or mating surface, by having each side of the tenon, or male segment, mating to the inside surfaces of the milled stile.  Double glue mass is good.  Particularly when assuming the adhesives are determined with a good sense of separating the hierarchy of the available choices in today's exterior glues.
Advantages:
1)  Double the mating surface--an improvement over the half-lap.
Disadvantages:
1)  We have already covered the issues with grain direction and seasonal breathing, and this joint is not exempt from the faults discussed in the above example. 
2)  We have also covered the issue of nothing between the top of the two joint seams and the falling weather.
3) And finally, the right side illustration resulting in our gate giving way, once again, to the madness of gravity.

  The traditional Blind Mortise and Tenon joint.  Here we have a male tenon milled from the same stock of the horizontal rail.  The tenon consequently mates to a corresponding mortise milled into the vertical stile.  Assuming a good tight fit has been fabricated, this is a reliable and long-standing favorite among woodworkers and furniture-makers.
Advantages:
1)  The top of the joint is concealed from the falling weather.
2)  There is what we call a 'shoulder' above the tenon, preventing, by a matter of simple physics, from the tenon dropping to the ravages of gravity.
Disadvantages:
1)  We are still wrestling with the issue of breathing in opposing directions.  Not so dramatically as our first two examples, but nevertheless we have a tenon that is an extension of the horizontal rail and this tenon is going to expand and contract in a direction that is perpendicular to that of the stile.
3)  The strength of our joint between the rail and the stile is now resting solely on the properties of the tenon, or western cedar.  Western cedar is not a dense wood, nor particularly suited for the stress loads of harder woods.  It is conceivable that the tenon could break, or snap.

For obvious reasons, the CPW joint is not illustrated. It might be as simple as bands of single and two-sided high-strength band-aids with colorful little actions figures, or as schematically advanced as one of the examples shown below.  Whatever our methods, the long-term structural integrity of the gate begins with the joinery.

Because Charles has written a number of articles over the years on the art of joinery for such publications as Fine Woodworking, Fine Homebuilding, Old House Journal, This Old House, and Woodwork Magazine, among others, it seems only appropriate to carry our discussion a step further, skipping ahead to a few advanced examples in an effort to illustrate not just the shere perplexity of this procedure,  but the alluring temptation of the joint as a work of art in itself.
Below is a look at an interlocking dovetail with stepped haunches, or shoulders.  A spline, essentially, that might, to the layman, more closely resemble aRubic's schematic but to the careful and discerning woodworker it becomes closer to performance art. 

Below, a look at the glue-less joint, where the interlocking planes and wedges are such that the joint is self sustaining.  The example here is often utilized in Japanese temples for joining the rafter tails to cross-beams.  Take notice of this on your next trip to a Japanese Tea Garden.

And then, below, an assortment of variations on the traditional mortise and tenon. 

As a small part of an extensive CPW interior design, the use of the traditional, yet modified, Finger Joint adds an alternative flavor to the standard Butt Joint more commonly used here.

Moving along, we arrive at the exposed joinery of a trestle table that employs both the Exposed Wedges of the Through-Mortise & Tenon Joint, but also the Primary Vertical Wedge joining the trestle board itself.   

Another look at a CPW Table using the Vertical Wedge to join the trestle board.
Joinery of this nature becomes a defining feature of the work's inherant design.

The hierarchy of joinery takes us to a series of intricate lamp joints, and the near-obsseive use of the joint as an ebony wood hinge. 

You are expecting an essay on Gate joinery, and instead have been lead on a meandering diatribe that ends with the portrait of a craftsman who often cuts the joints to your gates standing on his head, blindfolded, and one hand tied behind his back.