Let's simplify this measure task.
When measuring a window for an opening, it is width and height once.
This is the same for the arched window trim, but twice.
When the install is complete, it should look something like this.
The first measure asked for is the base width from inside the trim to inside the trim.
All Arched Window Trim order forms refer to the face of the trim as (A).
4 measures make up (A).
Then the height from the base of the half round area to under the trim.
If the arched window is atop a square window, place a line, level or string across to keep the base mark.
The base is always where the arch starts turning.
Now the same base measure of the window, but measure outside the window trim.
And the center height, this time from the bottom to over the top of the arch of the trim.
This is what makes up (A) for the face measures of the new cap.
Do not add or subtract any of these measures.
They rarely add up to a perfect half circle. This is expected.
If these are of existing wood trim, the new trim cap will be made to fit over the wood trim.
These measures will be transferred and made as the back of the new trim.
With the outside of the trim now being about 1/2" wider than the measures.
This allows for the cap thickness and 1/8" of wiggle room on both sides of the wood trim.
If there is no existing wood trim, the measures can be of the face of the new cap.
This will render the new trim the same width as the other window wraps on the project.
The depth (D) is in reference to the trim depth from the window, not the outside.
Do not wrap the rest of the window until the new Half Round Trim arrives.
Placing the new trim cap and then making the other wrap pieces to match works better.
New Arched Window Trim will have legs of 2" longer than ordered.
This allows for trimming and shaping to the install type.
Whether the window has a window seal or is connected to a rectangle window below.
Many options have been added to these half circle trim pieces for easier choosing.
One last thing.
To order Round Window Trim,
choose the trim style desired,
and enter "mull for round" in the comments box.
Text message 901.281.2887