Now, my dear readers, let us depart from the suit itself (for we are only giving an introduction and not attempting to explain every arcane detail), and delve into the mysterious process by which bolts of fabric are transformed into elegant (or not) suits. There are three, in descending order of price/snobbery: bespoke, made to measure ("MTM"), and off-the-rack ("OTR").
We'll split them into two posts, good readers, because I think bespoke deserves a bit more time and loving attention.
Now, it should come as no surprise to you, ladies and gentlemen, that I hold that bespoke is the ultimate expression of the suit. It is the Platonic ideal to which all other suits (MTM and OTR) are but pale reflections. Alas, it is almost impossible to find true bespoke here in Southeast Asia. Believe me, I've tried. I've all but resigned myself to flying to Hong Kong for my bespoke fix.
What, you may ask, is bespoke? And how do you distinguish it from its lesser cousin, MTM? First, bespoke means that a hand-drafted pattern will be cut for you by your tailor; he will take some 15 – 20 measurements and observations of you to start with, and then using those measurements and observations create on paper a unique pattern for your suit. He will offer you a choice of fabrics and colors, including the color of your jacket lining. He will be there to advise you on the cut of the suit and the details of the suit (lapels, pockets, vents, et cetera) and work with you to create a suit that is entirely yours. If he is good at his craft, he should be able to size you and recommend appropriate details and cuts to bring out the best in you. (However, I should add that in Asia, it seems that most tailors are less garrulous, and less eager to offer their advice. You may have to rely on yourself, or ask them specifically for their advice.)
No bespoke suit should be made without at least two fittings (unless you're a regular and your pattern hasn't changed much): the baste fitting and the forward fitting. Some bespoke tailors (though none that I have seen in Asia), use a third fitting called the fin bar fin fitting to check the fit one last time before putting in the buttonholes and other detailing.
The baste fitting, also called the "skeleton baste", is done after the pattern has been drafted and the fabric has been cut. At this stage, the pieces that will someday be your suit are sewn together quickly. There will be some effort to put in some of the "structure", such as canvas and padding (if necessary), so that one can get a basic feel for whether the pattern is a good fit for you.
I've noticed that a lot of Asian bespoke tailors skip the baste fitting, incidentally (including Mr. Chan of W.W. Chan & Sons in Hong Kong). Some Saville Row tailors will also skip the baste fitting, notably Anderson & Sheppard (who used to dress the Prince of Wales as I recall rightly). I personally prefer to have a baste fitting, because it offers a bit more leeway to correct any errors in the suit before we get to the forward. If you are doing a dinner jacket or a morning coat, gentlemen (or ladies on behalf of their gentlemen), please insist on a baste fitting. Dinner jackets and morning coats are incredibly finicky, and you'll want to give your tailor every opportunity to catch issues before they balloon into serious problems.
After the baste fitting, the tailor then deconstructs the suit, makes any necessary alterations, and starts actually putting it together the suit in its finished form.
The forward fitting is the most important fitting. At this stage, your suit should be mostly done. The pockets should have been cut; the canvas lining (to give it structure) should have been sewn into the jacket; and the suit should start to look like a suit rather than a collection of pieces of cloth hastily stitched together with white thread. At this stage, the sleeves will not have been fully sewn in (because this often needs to be adjusted), and the lapels and collar will not be fitted and finished.
After this, the suit is then finished: the sleeves are properly attached, the lapel is hand rolled to give it that elegant shape that distinguishes bespoke from mere MTM and OTR.
This, my dear readers, is the bespoke suit. It is expensive (my bespoke suits from Hong Kong cost around HKD8,000 or approximately USD1,000). It is labor intensive (ideally most things should be done by hand). It is not for everyone. But if you want the best possible suit, with the most attention to detail, you can only turn to bespoke.
Update (08/08/2010): Primus Sartorius has been informed by his excellent tailor at W.W. Chan & Sons that a baste fitting is the norm for their bespoke suits, but sometimes can be dispensed with in the interest of time if it will not compromise the quality of the finished product, or where the customer is a regular whose pattern has already been refined and is not at risk.