Dividers let you lay down the important points in your design. The straight edge connects the dots to give your layout form. I know it seems a little obvious that you need a straight edge for drawing straight lines, but its important enough to me that I felt the need to discuss it here.
There are generally four camps when it comes to straight edges. There is the “whatever long thing is close at hand” camp. Sometimes its straight, maybe. I admit that I lived in this camp most of my life. On the other end of the spectrum is the camp that has to have precision ground and certified straight edges. There is nothing wrong with this camp if that level of accuracy is what gives you the warm and fuzzies. The other two 
camps lay somewhere on the middle ground. One camp contains a somewhat fancy stick and here is a link to an article that about how to make it. There is a little too much emphasis on finding the ‘perfect’ piece of wood and its a little fussy for my taste. The last camp, where I now reside, has a purpose built stick that is trued, maybe decorated, but is still just a simple stick. Chris Schwarz posted about this type after seeing examples crop up in several historical paintings and engravings.
That historic version struck a cord with me and I went out to the shop and built myself a straight stick. Not exactly my most glorious moment in woodworking but it was a step towards a more efficient workshop. No longer was I hunting around for something long enough to connect two points. It was right there hanging on the wall. I’ve also use it as a batten and fence by screwing it down to my bench when the need arises. Mine is made from pine and was finished with a little bees wax applied with the polissoir. I find that a dedicated and slightly refined straightedge is a very handy shop appliance and cheap to add to your tool kit. I created an illustration that covers the main points and shows a couple of layout methods for adding a little decoration and function to the ends. Have a look and enjoy.
Greg Merritt
HILLBILLY DAIKU 
Interesting piece on the subject and your illustration/print is a great aid. Perhaps sometime you could show us how to do more curvey line designs with the dividers. I would bet that I’m not the only one who doesn’t have much idea what dividers are for.
Cheers
That’s where I’m heading with this Randy. I’m just laying the foundation before I start the building. I hope that you are enjoying the information so far.
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On straightening the edge: make two sticks instead of one. Shoot them both at the same time, laying the plane on its side. Compare them to each other. Looking for light, on the order of a couple of thousandths of an inch, is way more precise than matching up to a scribed line, and just as quick.