In the very first Connecticut houses, architectural detail was minimalistic and strictly utilitarian. But even the earliest colonists wanted their homes to look good, so in time, both interior and exterior detailing began to take hold. Much of this elaboration incorporated both Classical and Jacobean forms. And even prior to the middle of the eighteenth century, arriving English carpenters had begun infusing colonial architecture with the prevailing nuances of Georgian England, and the teachings of Christopher Wren.

A beautiful example of exterior architectural detailing surrounds the front entry of the Edward Curtis House (c. 1745) of Stratford, Connecticut. It features fluted pilasters supporting a cornice with a dentil band. Carved Tudor (6-petaled) roses decorate their capitals. The very thick lights at the top of the paneled doors are known as "bulls eye" glass, and are also found in the front door of the nearby Judson House.
Colonial carpenters sometimes added their own unique flourishes, and often you’ll find variations of the same basic architectural theme in different locations. For example, the fireplace paneling in the parlor chamber of the Rev. Richard Mansfield House (c. 1700), about fifteen miles away in Ansonia, Connecticut, includes elements nearly identical to those detailing the entrance of the Curtis home:

The fireplace paneling of the Mansfield House parlor chamber likewise features fluted pilasters, Tudor Roses, and breaks in the upper bed molding similar to the Curtis home entrance. The central composition of panels, rails, and stiles, situated between two pilasters as it is, is reminiscent of paneled doors. Whether this was done intentionally, or not, is probably impossible to say.
In recent times, I’ve become obsessed with colonial architectural detail, and its evolution within the Connecticut and New Haven colonies, and their surrounding regions. In particular, I’m determined to capture as many surviving examples as possible, both in photographs, as well as in three dimensional models drawn using Trimble SketchUp. I’d also like to establish an online archive of architectural models unique to our region.
To this end, I’ve selected another example from the Mansfield House for an initial survey project — the fireplace paneling of the hall chamber, which is rather nicely executed, but also sufficiently straight-forward to make for a reasonable first attempt:

Fireplace paneling of the Mansfield House hall chamber.
As often was the case in early Connecticut homes, the fireplace paneling here is integrated with the chimney girt and rear post, stylistically “supporting” the girt as a simple cornice or entablature, via a bed molding. This particular fireplace paneling sports a number of interesting architectural features, including this rather elaborately built-up mantel shelf:

Mantel shelf.
Also of interest is the heavy bolection (sometimes called Italian) molding surrounding the fireplace opening, while the panels themselves are beveled, and rabbeted into beaded rails and stiles:

Heavy bolection molding and raised, beveled panels.
So, to start off as simply as possible, I decided to initially model the bed molding beneath the chimney girt (excluding, just for now, the small cove supporting it), which is of classical cyma recta contour, and includes a mitered return at the end opposite the post:

Cyma recta bed molding "supporting" the cased chimney girt.
The other end of the molding terminates flat against the rear post’s casing:

Left end of the bed molding.
I collected contours from three different locations along the bed molding (all were slightly different), and standardized on what I’d hoped was a reasonable interpretation, on my part, of what the original craftsman had intended:

A head-on view of the profile of the bed molding, revealing its slightly distorted shape.
I did so using a standard contour gauge:

My contour gauge and notebook.
And transferred them as best I could to my engineering notebook. I also measured and recorded all of the lineal dimensions of the molding as accurately as I could (a pair of very long dividers comes in handy when measuring irregularly shaped or obstructed features):

Some of the contours and dimensions captured in my notes.
Once I felt reasonably confident in my measurements, I set about building an initial SketchUp model of the molding, by first drawing the rectilinear segments of the molding contour in two dimensions, and adding guidelines corresponding to the vertical graph lines of my notebook:

Initial model in two dimensions: Rectilinear profile segments and guidelines.
Next, I added horizontal guidelines corresponding to the points where my captured cyma recta curve intersected with the vertical lines. I then used the SketchUp arc drawing tool to fit as smooth a curve as possible between these intersections:

Initial model in two dimensions: Horizontal guidelines delineating cyma recta contour intersections with the vertical guidelines.
The final, two-dimensional representation of the contour looked like this:

Initial model in two dimensions: Molding profile.
Using SketchUp’s pull tool, I then extruded the two-dimensional contour upward along the third dimension:

Initial model in three dimensions, with the molding contour extruded upwards. The vertical lines are a side effect of how SketchUp manages curved surfaces, and are easy enough to hide.
Then, I “flipped over” and rotated this three dimensional shape so as to properly align it with the standard axes defined by SketchUp. Doing this ensures that modeled components are correctly oriented when combined together to build more complex models. I also lengthened it a bit. Here’s what the resulting molding section looked like:

Cyma recta bed molding section in three dimensions.
Now that a basic molding section had been created, my next big step was to figure out how to miter either end. Unlike real molding, I couldn’t take a double-bevel compound miter saw and simply cut it — that would’ve be too easy! Instead, I had to figure out how to “cut” a 45 degree miter in SketchUp. SketchUp is a bit rigorous about what you can and can’t do when altering irregular shapes. Admittedly, I had to try this a few times before I finally got it right. What follows are screen shots of the steps I performed, in the event this is useful to others attempting to do the same thing (if you’re not, feel free to skip over the impending tedium).
The first step was to rotate the molding section so as to view its back side, and draw a 45 degree guideline across its top:

Molding section with 45 degree guideline.
Then, I “scored” a vertical line down the backside of the molding section, beginning at the miter line:

Vertical line drawn from miter line down.
Next, I selected the near vertical edge, and, making sure that nothing else in the model was also selected, attached the SketchUp move tool to the top corner of that vertical edge, and “swung” this vertical edge over to meet the miter line. The end result looked like this:

Result of moving the near vertical edge to the miter line.
The resulting white appendage seen above is the projection of the other side of the contour into the three-dimensional solid, and it now needed to be carefully removed by “intersecting” it with the remaining solid, and then “subtracting” it away. To accomplish this, I first drew a solid line along the bottom edge of the white geometry:

Solid line drawn along bottom edge.
Then, using ctrl-left-click, successively selected each of the curved contour sections (remember those vertical lines I needed to hide earlier, after I’d first pulled the two-dimensional contour upwards into the third dimension?). In the screen capture shown below, the top two or three contour sections have been selected (indicated by the slightly greyed-out areas):

Contour sections in the process of being selected.
Next, I used SketchUp’s intersect faces with model operation to effectively divide this section from the main model. Once this operation is actually performed, the curved line where the contour meets the mitered wall goes from transparent to solid:

Intersecting the faces of the contoured section with the rest of the model.
Now that the model had effectively been divided, the unwanted portion needed to be deleted. To do this, I rotated the section back the other way, selected the main edges defining the separated contour, and then performed an erase operation:

Selecting and deleting the separated contour section.
The end result of this was a cyma recta section with a 45 degree mitered end:

Cyma recta section with mitered end. The final step is to erase the remaining guideline.
I then repeated the same steps in creating another molding section, but one with a 45 degree miter on the opposite end:

Cyma recta molding section with other end mitered.
As a result, I now had both “left-hand” and “right-hand” mitered sections, saved as SketchUp components that I could readily import into a model and join together to form corners:

Corner formed by joining two oppositely mitered ends of molding sections.
Finally, by extending the non-mitered end of a “left-hand” section out to full length (in this case, 89-3/4″), and appropriately shortening a “right-hand” section to model the return, and then joining them together, I created an accurate model of the bed molding beneath the chimney girt in the Mansfield House’s hall chamber:

View of the modeled bed molding from below.
This second view of the same model reveals more clearly how the return had been cut from a separate piece of wood, and simply joined to the mitered end of the main piece:

View of the modeled bed molding from above and behind.
Conclusion
There’s no doubt that modeling historic architectural detail in this manner is a lot of effort. But it’s time worth spent for anyone serious about capturing this information and making it readily accessible to others. The educational advantages of three-dimensional, digital models are significant: one can readily view, explore, rotate, and deconstruct such models to learn more about them. Also, if arbitrary two-dimensional plans or sections are desired, they can always be produced directly from the same three-dimensional model, without the need to create additional diagrams.
Furthermore, once a library of standard components has been established, new models can readily be created by piecing existing components together, and creating customized versions of those components wherever necessary. Models formulated using Trimble SketchUp can easily be published on the Internet via the Trimble 3D Warehouse. For example, both the left and right cyma recta sections I’ve created here may readily be downloaded from my own Trimble 3D Warehouse page. They can then be viewed using either Trimble SketchUp or Trimble SketchUp Viewer.
Finally, yet another advantage to publishing archives of models of historic artifacts online is that well-established SEO techniques can be leveraged to ensure that these model catalogs are found by those searching for them, while social media can likewise be used to publicize the existence of these archives to their intended audiences and communities.
Postscript
This modeling exercise of mine was (very gently) criticized, recently, via Twitter, on the claim that there was no obvious, practical need for three-dimensional models of historic architectural millwork. While I don’t agree with that claim, I do understand the basis for it.
If I were creating a collection of SketchUp components representing well-known classical forms, or even standard millwork, then I’d happily concede that my critic had a point, as these forms are widely understood, and a great many examples of them have already been published in the SketchUp 3D Warehouse. But what I’m doing here is capturing the architectural details of specific historic buildings, and as such, I consider each modeled element to be fundamentally unique, even if it expresses some well-known shape.
For example, there’s nothing particularly profound (in a more general sense) about the model of the bed molding that I’d developed and illustrated through out the course of this article. But this small component will soon become part of a larger model of the entire paneled composition, which itself is quite unique, and of considerable historical significance. In that sense, even a trivial piece of molding needs to be accurately represented here. So I’m not inclined to search the 3D Warehouse for close equivalents, but rather model these pieces myself, and directly from my own measurements of their real world prototypes.
I should also point out that the models I’m constructing and publishing are primarily of regional historic interest. Some one researching the habits of early Connecticut carpenters might find them invaluable; but general architectural historians, perhaps somewhat less so (or maybe not). These are points I didn’t make completely clear earlier, especially in the above Summary, which seems to suggest more general intentions.
And finally, a secondary objective of the article itself is simply to share my own experiences using SketchUp with others who are undertaking similar efforts.
Post-Postscript
Here’s a photo of another cyma recta bed molding from the Mansfield House, this one from the hall fireplace. A small lower section missing from the return reveals that the molding is shaped from an angled board, as initially suggested in a comment posted by Jane Radocchia, and subsequently discussed by Sebastian Eggert:

Opened cyma recta bed molding from the hall fireplace of the Mansfield house. The small supporting cove also appears to have been shaped from a separate piece of wood.
This molding is of the same contour and dimensions as its counter part from the hall chamber, and I have no reason to believe them to have been constructed differently. So as soon as I’ve had a chance to measure the section, I’ll revise my earlier model to reflect this shape. And that same model will be used to represent either bed molding. So much thanks to both Jane and Sebastian for encouraging me to consider this.
Also of interest is the contour shown below, which was recorded by famed New Haven, Connecticut restoration architect and historian J. Frederick Kelly, on p. 192 of his “Early Domestic Architecture of Connecticut” (published in 1924). Kelly cited it as an example of a non-Classical contour that was indigenous to Connecticut, and often found in later period (Revolutionary to Greek Rival) compositions through out the state. So I’ll be keeping an eye out for this one in my travels, as well.