When laid flat, the pieces of the typical pre-1820 tail coat are shaped basically the same as a coat of the late 18th Century but with growth across the front and cutaways in the lower front bodies to form tails (plus various other evolutions). The diagram of this coat on the attached link shows the beginnings of the evolution- the front body has barely been formed into tails in this 1790-95 example:
Click for flat diagram: http://www.lacma.org/sites/default/files/FF_Patterns_Manscoat.pdf
The diagram of this coat from ca. 1812-14 shows how the coat body typically evolved from there; the basic form lasted through the first two decades (and sometimes beyond) of the 19th Century:
Diagram by Betsy Bashore:
The body was still cut in one piece as it had been since it took hold as the typical male top layer in the 1680s (elaborated on nicely in Part 2 of Noah Waugh's The Cut of Men's Clothes 1600-1900).
This remained the case until about 1818 when modifications began to be made to tighten the fit of the body, which would ultimately lead to the pinched waist, flared bottom look that would take hold in the 1820s and last (in changing forms over time) through the 1840s, into the 1850s.
ca. 1840s- MET Museum
The first step toward this was the introduction of a "fish" at the waist, which began the "tightening trend" (first appearing in 1818 according to Waugh, 113; indeed, a thorough survey of reliably dated period images corroborate that there is no evidence of change before then).
In the 1820s, waist seams and body darts began the process of being introduced (although not universally) to aid in the image created by coats such as those in the images above. The Tailor's Friendly Instructor by J. Wyatt from 1822 is an interesting look at this evolution in progress: the structure of the front body of his tail coat diagram is still cut in one piece with only the frock coat possessing a seam at the waist and dart in the body.
Tail coats seem to be misunderstood by those attempting to recreate the Regency/Federal period today most commonly because of a "projecting backward" of what has been understood to "be" a tailcoat since just prior to the Victorian era: waist seams and body darts will creep in on garments intended to represent an era when they did not exist, creating a fit and silhouette that also did not exist (an error which I made, myself, for years). The tailors of the Regency/Federal period only had knowledge of the developments that had been made to that therefore only had that to work with. It is also important to keep in mind that over the 30 or so years that are often lumped into the label "Regency," many trends came and went within the time frame, so limiting research for a coat to no more than a five year span is important.
I am often asked what pattern on the market is best for making a "Regency" coat. The best solution I have found is to take this pattern and "boil it down" to its basic shape (eliminate all of the military details). You will then have a basic template to work from; the front edges as is will meet along your center front (overlap for buttons/holes must be added). You can then build the civilian details onto the basic shape too create the specific civilian look you desire (the sleeves ARE supposed to reach the knuckles; fiddling with where the body cuts across to form the tails will likely be necessary):
I know more than one person (myself included) that had to take an inch or more out of the side seems to achieve a good snug fit for their size but, other than that, it works out beautifully. It is important to keep in mind that items representing later periods are often mislabeled "Regency" and even assigned a date range that literally would include the Regency era. This is an example that applies the term "Regency" and recommends it for recreating the years 1810-30 but, when the fit, details, and diagram are checked against primary sources and the evolution outlined here, it is actually not correct until the 1820s are well under way and would actually be a very useful resource for recreating fashions into the 1830s (rather than cutting it off at 1830).
Recreating early tailcoats can be a tricky question but understanding how they evolved and doing your own research based in primary sources limited to a five year span or so before making any purchases, it can be done. My most recent attempt, ca. 1805-10, based on the process I recommended above:
The Victorian Man (Looking Backwards)