In this post I discuss the American "third line" at Guilford Courthouse -- the part of the battlefield where the American Continentals were deployed and where the Americans hoped to stop the British attack.
Visitors to Guilford Courthouse National Military Park (website) are probably aware that there has been a certain amount of controversy concerning where the third line was stationed. Some years ago, the National Park Service concluded that the area in which several monuments had been placed was not in fact the correct location. The "old" and "new" third line sites are roughly indicated below.
The map at left is from the one that I prepared for this project; the fields were drawn in such a way as to be consistent with the new interpretation.
Participant accounts strongly support the new interpretation. Here is Lieutenant-Colonel Henry Lee’s description of third line area:
“Guilford court house, erected near the great state road, is situated on the brow of a declivity, which descends gradually with an undulating slope for about a half mile. It terminates in a small vale, intersected by a rivulet” 
The courthouse is near the junction of the two roads on the map. The "rivulet" presumably what is today called Hunting Creek, the stream that crosses the width of the map. His description goes on to place the open ground on which much of the third line fighting took place as between the courthouse and the rivulet, on either side of the road.
The new third line position is on a hill that is much higher and broader than the old third line position. It more clearly can accommodate the two brigades of Continentals that fought at Guilford Courthouse.
At the beginning of the battle, the third line consisted of two Virginia regiments that averaged about 385 men each (both officers and enlisted men), two Maryland regiments that averaged about 340 men each, and a two-gun battery commanded by Captain Ebenezer Finley. The Virginia regiments formed a brigade on the right of the line, the Maryland regiments formed a brigade on the left of the line, and Finley’s battery was located roughly between them .
Maps of the third line at Guilford Courthouse usually present the American deployment in a simplified manner: either a simple straight line or four closely aligned rectangles (one for each Maryland and Virginia regiment). However, the source material allows for a more precise understanding of how each regiment was positioned. Some key passages are quoted below. 
- Major-General Nathanael Greene: the Continentals presented “a double front, as the hill drew to a point where they were posted, and on the right and left were two old fields.”
- Lieutenant-Colonel Banastre Tarleton: “The flanks [of the two Continental brigades] did not dress up to the center, but were drawn back, so that each brigade presented a different front…”
- Lieutenant-Colonel John Eager Howard: "the first [Maryland] regiment... was formed in a hollow, in the wood, and to the right [west] of the cleared ground about the Court house." "...my station [was] on the left of the first regiment, and next the cleared ground..."
- Lieutenant-Colonel John Eager Howard: "The second [Maryland] regiment was at some distance to the left of the first, in the cleared ground, with its left flank thrown back, so as to form a line almost at right angles with the first regiment."
The exact spacing of the troops on the third line is not known, but a “close order” formation seems likely. As three of the four regiments were in a wooded area, it does not seem likely that the troops were packed closely together (which anyway does not seem to have been the norm by the late war; see this website for a lengthy discussion). A reasonable guess, I think, is that 12 inches separated each man. If correct, than the Virginia regiments had a front of around 480 feet each, and the Maryland regiments had a front of around 425 feet each. 
Taking all these points together, I believe the regiments were deployed on or near the spots indicated on the map below. The blue lines represent, from left to right, the 1st Virginia, and 2nd Virginia, the 1st Maryland, and the 2nd Maryland. The two blue “t”s represent the two cannon in Finley’s battery. The orange lines represent watercourses or ravines. Note that the left and right flanks may have been anchored on ravines. The historical New Garden Road roughly corresponds with the road marked in red-and-white that runs from the top of the map to the bottom. Notably, there may have been a gap at this point in the American line – perhaps to accommodate the retreat of the first and second American lines. 
Here is what the regiments look like on the battlefield map I created.
1. Henry Lee. (1812). Memoirs of the war in the Southern Department of the United States
2. I discussed the American order of battle here. To the best of my knowledge, the source material does not directly indicate where Finley’s battery was located, although there are certain clues. Based on these, Lawrence E. Babits & Joshua B. Howard (2009) in Long, obstinate, and bloody: The battle of Guilford Courthouse concluded that "Finley's two six-pounders were situated on the terrace's southwest corner" where they covered the most probable British routes of attack (p. 144). I believe this interpretation is probably correct, but there are minor differences in our maps.
3. A useful compendium of sources for the battle of Guilford Courthouse can be found here. Howard’s account is quoted in Babits & Howard, ibid. and in James Herring and James Barton Longacre’s (1835) The national portrait gallery of distinguished Americans, Vol. 2
4. Here is the very crude math: 18 inches (presumed width of a typical 18th Century soldier) + 12 inches (as described above, the assumed spacing between soldiers) * 385 (estimated men in 1st Virginia) / 2 (usual number of ranks) = 5775 inches, or 481.25 feet.
5. Worth noting is that the interpretation presented here is similar to that described by Babits & Howard, ibid. The only real difference in interpretation, as best I can tell, concerns how much of the 2nd Maryland should be placed south of the New Garden Road (they suggest most, I suggest all). As is so often the case, the source material lends itself to more than one interpretation. To imagine what the line would look like according to their account, one should mentally move the 2nd Maryland regiment north until the right flank of the regiment crosses the New Garden Road.