Why not run your rails along the front of the posts, attaching with nails and then attach the boards to the rails.......no need for holes in posts.....or am i missing the question you're actually asking?!?!? :lol:
I would suggest using chamfered rails (or square)and surface mounting them to the posts, of course you will have to set the posts back from the boundry so that the o/s face of the featheredge is on the boundry.
Alternativly if you have to use triangular arris rails ether cut a flat on rails where they meet the posts and attach as above, or attach using screws or nails driven in at 45 degrees through the end of the rails (scew nailed) or use a "feine" or "bosch" oscilating plunge cutter to cut the holes or resort to drill and stitch method as mentioned in a previous reply.
The "saw a triangular hole" in the face of the post will effectivly half the strength of the post. :(
You can buy galvanised triangular arris rail brackets so that you do not need to cut hole for the rails.Fit the uprights and 2 arris rails between and then nail the brackets to each end of arris rail and nail into upright.
This is a very old post, we are aware of that but we have become aware that it is still viewed many times every day so I thought I'd take a look. There are no rules anywhere which say an arris rail has to be used and it is perfectly acceptable to use square edged timber to form the rails on any type of fence. The attraction of arris rails is that the water runs off the sloped back, but the reality is that when a fence is installed with square edged timber the chance of the top edge being 100% level are tiny so the water problem takes care of itself. To see how to install close board fencing, please go to this link. http://www.diydoctor.org.uk/projects/closeboard.htm