Marriage marks (sometimes called carpenter's marks) are markings cut into the timbers of a timber frame to indicate where they are located in the building. Think of them as labels to show the carpenters where the piece goes when they get ready to put the frame together.
Marriage marks are mostly associated with frames that were scribed together -- each piece being custom fitted to its exact location, and therefore different than the other similar pieces. In scribe frames, it's common for each timber at a specific intersection to be labeled with it's own unique mark as you can see in many of these photos.
Here's an example from an 1850's building in Graniteville, SC.
You'll also notice that the marks are Roman numerals. That's the most commonly seen labeling system. It's faster and easier tor cut the straight lines of Roman numerals with a chisel or race knife as opposed to the curving Arabic numerals such as 8 and 5. Often times a "flag" or other modifier would indicate a specific side of a building or floor level. The use of modifiers keeps the carpenter from using numbers like LXXVIII -- Each wall might start at I and the north wall may have a flag left while the south wall has a flag right. Or perhaps the north wall is cut with a 2" chisel and the south wall a 1" chisel.
There is an incredible variation among marriage marks from one region or country to the next -- and even within the same area you can see several different systems that were used at the same time. Unfortunately, we don't have a comprehensive understanding of how many different systems were used or their distribution even in any one country. Anyone looking for a topic for their master's thesis?
I've included a few photos from buildings we've worked on, built, or studied.
1670 era indentured servant's quarters
at Charles Towne Landing State Park near Charleston, SC.