Comb graves are tent-like burial markers, which are generally created from two long pieces of flagstone laid next to one another long-ways, with a headstone and foot-stone (as seen in the above photo). This grave type is local to the Upper Cumberland region. Dr. Ric Finch, in his upcoming article, "The Tennessee Comb Grave Tradition" in the Tennessee Folklore Society Bulletin provides great detail in what is known about this style of burial.
The Tennessee Comb Grave Tradition - Figure 1 color variant
The Tennessee Comb Grave Tradition - Full page interactive map
For more information, see Ric Finch's webpage, GrateRutabaga.com, and review the many publications he has available on the topic.
For further reading...
Comb Graves of Tennessee by Tennessee State Library and Archives
Digital Collection: Comb Graves of Tennessee
My photographs of comb graves
Below is an interactive map created using ArcGIS Online and the data Dr. Finch provided while I developed maps for the article.