I was putting off replieing in hopes that someone else from common purlin land might pipe in...

It would be kinda cool if the forum could be used almost as a research tool, and some greater sense of the extent of the range of dominance and varitions of historical example might be posted.

But I'm thinking there are too few regular posters or even lurkers from the area for any patterns to emerge...

There are three common variations I see most frequently, I'll start with what is probably the least common of the three, because it has a tenoned member, while tenons are used infrequently in CP systems. I've seen the term Major Purlin / Minor Purlin bantied about in print, to describe (if memory serves) this version, (though I've never used it in conversation) I think sussex Oak might know it as a Butt Purlin (on the road at the moment so I connot consult my library) I know the main tenoned purlin as a Principal Purlin (but that may be confusing to some in that this term is often used interchangebly with Purlin Plate)

In this variation a heavier (Say 6 X 9 on the flat) tenoned purlin is found at midpoint of the Principal Rafter - This almost always found with up & down wind bracing - Sometimes they are staggered up & down (to allow for greater tenon relish) - I have seen the occasional in-line example where two hi/lo bypassing tenons shared the same mortise.

Perhaps the most common variation has regularly spaced (40" - 42" - Memories not serving - I should know, but this is only an educated guess) purlins - These at first glance, appear to be individual pieces, (This is noticed most easily when round poles hewn on the top and adzed to square at the Principal Rafters are used as CP's) but often span multiple bays (two most commonly) and the breaks are staggered with intent - a technique which survived the transition to CG framing (and indeed became more important and necessary) - These purlins like the ones above and below the Principal in the above example, lap unbroken through a two inch deep dap in the top surface of the Principal Rafter.

There are examples from both sides of the transition which are broken at every bay. Like the ends of multiple bay purlins, two opposing laps share a dap. In both cases the rest/bottom of the purlin sits in a housing.

The last common variation, is found most commonly in earlier scribed buildings, and has staggered purlins sharing a common dap, with a short section (10 to 18 inches) for relish dropping down to full depth in the bay beyond. These also span multiple bays.

Don -- I have never seen a historical CP which was dovetailed, (Though they do drop in) but it was a common way of joining them early in the current revival.

Are the pegs you are seeing a single peg in the center of the rafter ? Or are there groups ?