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  Jack Miter Jig  
My thanks to Jim Chestnut and Jeff Burks for introducing me to this great method for installing quirk-and-bead casing (or other simple beaded colonial styles). In answer to Dan DeVol's question about New England-style trim, Jim wrote (for the entire subject, see JLC Finish Carp. forum: New England Home Details, and Jack Miter Progress:)

Dan, You are a man after my own heart - and that of many trim carpenters in New England.

The casing you describe is an excellent choice - with or without plinths one piece or two ( ie. separate backband or one molded as a part of the rest).

In my opinion, the biggest advantage to that style casing is its versatility. If two piece, the bead can be jackmitered, the rest of the casing butted, and the back band mitered. The jack mitered legs can be pocket screwed together prior to the back band being installed. You will have zero problems with wood movement using this method, and is the way it was done in colonial times.

Another advantage is that you can vary the widths of the flats to make things fit properly - which is not possible with highly detailed profiles. For instance, if a cabinet starts a little too close to a window or a door, the trim guy can use an entire casing that may be slightly smaller than the rest in the house - but no one would ever know.

Upstairs, where the closets are jammed into the corner where the bedroom door is, a bead can be run on flat stock with a router (Whiteside makes beading bits in increments of 1/32 radii), and the two flats (the adjacent closet leg and door leg) can be jack mitered into the head casing. Then the back band runs up one door leg across the head casings of both door and closet continuously, then down the furthest closet leg , all in one continuous piece. It then looks as if the architect knew what he was doing. You will be able to maximize much desired wall space without sacrificing the aesthetic integrity of the room.

Though it may cost you a little more to do two piece, that is the way it used to be done, and the way I prefer to do it as well (despite the fact that miter clamps are not used).

If you have window walls, adjacent legs can also be easily jack mitered, making a series of windows appear to be a single unit. This can be done with one piece casing as well as two piece- if I haven't made that clear.  
This was the jig I imagined making, after reading Jim Chestnut's post, and looking at the photos Jeff B. posted.
At first, I thought I'd be able to install a stop under the jig, but that didn't work out. I did shoot a finish nail through the jig, right where that stop is, to help secure the jig to the casing.
Preparing to mortise a leg in the center of a head, I scored a line on both sides of a square-cut block

with a utility knife, so the line would be sharp and perfectly flush with the shoulder

I darkened the scored lines with a sharp pencil so they'd be visible in the photograh, and I highlighted the previous chamfer cuts in my jig, so it would be easier to line up the scored mark.
I shot an 18ga 1 1/4 in. nail through both sides of the jig, centered above the casing stock, then clipped off the nail leaving about 1/8 in. protruding from the bottom of the jig. I tapped the jig snug against the casing stock, which seated the nail so the material wouldn't slip while I was routing.
The clamps held everything secure while I ran the router. They also helped secure the stock in the jig.
There was a lot of chatter with this bit--I was taking a pretty heavy bite all at once. A plunge router might be better, with two or three pre-set stops. But the template guide worked perfectly. I ground the bit with my 4 in. grinder, so the bottom wasn't
exactly pretty. You can see a scar on the back side of one cutter, where the grinder slipped.
But the cutter still cut a flat-bottom mortise. For this mortise, I had to cut one side at a time, then move the material up against the opposite shoulder of the jig.
I made the jig wider than necessary, so I could do two 'rails' at once. That extra width proved to be very useful, as the two spacers you see fastened to the inside rails of the jig allowed me to see exactly where the bit would cut.
Here I'm lining up the jig for the second cut. The pencil line crosses the casing and marks the farthest cut of the bit, too. You can just make out where the radius of the bit ends, so alignment was a snap.
Sometimes the mortise wasn't cut perfectly clean on the bottom--probably because I used scraps to build the jig and they weren't consistently sized.
I fine-tuned the mortise with a sanding block. One side has 80 grit, the other 120 girt. Without a block, I would have rounded the mortise for sure.
This one could have been tighter. An early attempt. But I also found that shaving a hair off the end of the leg, or sanding the mortise a little more, helped tighten up the miter joints. Also, these pieces are just standing there. I didn't fasten them. A couple pocket screws and some glue would probably do a lot for this joint. And that's the point of this joint: pocket screws
add considerable strength and are easy to install on the backside of the casing.
Here's a typical head that's mortised on the end for a leg. The mark on the left registers the piece in the jig pefectly, so the leg will end up flush with the endgrain on the head.
Here's the leg-to-head joint. The jig really makes this joint simple. Henry P., this miter isn't screwed together either. I cut the ends of the legs on my mitersaw, first cutting the miter, then cutting most of the miter off square. I make a simple registration mark on the saw fence and on the saw base, so it was easy to cut the legs the right length. This jack-miter
joint is much stronger and more durable than a miter made across 3 1/2 in. casing stock. The angle of a miter cut across wide stock will change as the width of the casing expands and contracts with seasonal humidy/temperature variations (see Open Miters). The small miters pictured here won't be affected by seasonal wood movement. The butt joint might telegraph through the paint-- though pocket screws make that unlikely, but the small miter will never open.
This late 18th century kitchen (Abel Woods House; Sharon,CT) has a typical colonial mantelpiece. The backboard is constructed from varying width boards, and a simple plant-on molding frames the firebox and the oven/wood storage compartments,
The pilaster legs vary in width, and join the frieze with butt joints. The extreme width of the material, and the varying sizes, rule out miter joints.
The quirk-and-bead detail is jack-mitered, illustrating how useful the technique can be. As Jim Chestnut said, combined, jack-miters and pocket screws can be a carpenter's best choice, for design and durability.
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