I found this article on THE OLD HOUSE GUY and wanted to share it: Here's the full article: https://www.oldhouseguy.com/moldings/
I have read many blog articles about how to improve a house with the use of molding.
Adding a cornice with cove molding, embellishing window and door casings etc. are some examples.
These blogs say to take different types of moldings and put them together to create a more ornate appearance.
This all sounds great and at first glance it all looks pretty good too. But this is the wrong way to do it.
There are various types of moldings available and each has a particular use. As Brent Hull explains it “There is a language to classical design. If you don’t speak the language, your work will look funny and awkward. Put simply: You might be using the right words, but if you put them in the wrong order or upside down, they won’t make sense.”
Classical design is a very involved subject and I go into it in a little more detail on my CURB APPEAL page. Here however we will focus on two types of molding that are frequently used incorrectly. Once you understand them you will be able to recognize and pick out their improper use.
Supporting vs. Terminating Moldings
There are two types of moldings you should be familiar with. They are used very often and 
most times you will see they are used incorrectly.
They are Terminating moldings and Supporting moldings.
The Cavetto is a terminating molding.
It is to be used at the top of other trim showing the design is ending.
It is concave. Think of reaching your arms up over your head with your hands open.
Your hands are terminating the form of your body and your hands are facing forward in an open position as in the shape of the Cavetto (“C”).
The Ovolo is a supporting molding. 
It is to be used below something it is supporting like a shelf.
It is convex. Think of your arms stretched in front of you holding a book.
Your palms are facing upward and your fingers are curled in the shape of the Ovolo.
Cyma Recta has both the Cavetto and Ovolo combined.
Since the Cavetto is on top this is a terminating molding. 
This is usually seen as a cornice around your ceiling.
Cyma Reversa has both the Cavetto and the Ovolo combined but they are reversed with the Ovolo on top.
Since the Ovolo is on top this is a supporting molding.
This would be seen supporting a book shelf or something heavy.
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