For the overall look of the mandolin, I started with one key feature and designed from there. This feature is the smooth transition from the sides of the mandolin to the heel of the neck. Although this design allowed for a very simple mortise and tenon neck joint to attach the neck, it also made for some interesting challenges, ie the sides.
Since the sides meet the heel, and the heel is angled as a f or an a style mandolin is, the sides also have to bend at an angle. The other elements of the design flowed from this one main element. The body was meant to flow from the neck in one fluid line to complement fluid shape of the sides. The sound holes where added in such a way as to accentuate the curves of the body.
As far a acoustics, I tried to keep everything simple as I could and design in conventional ways which are well tested on similar instruments. The bracing pattern consists of a x brace on the top and a ladder bracing pattern on the back as is comonly used on mandolins and, in a more complex form, on guitars.
I chose to go with a four string mandolin mostly so I could try finger style playing similar to classical or acoustic guitar without the complication of more strings. The four string set up makes for a interesting alternative to some of the more traditional styles of playing. This mandolin could be set up with eight strings, but it would need a larger peg head to accomodate the extra tuners, four more tailpiece pins and a truss rod to handle the extra string tension.
Everything about the design was essentially laid out prior to building, and surprisingly, little of it changed when I started to make the actual mandolin. The one part that did change (and it changed more than once in the course of a week) was the tailpiece. I knew I didn't want to go with a standard tailpiece. I was originally going to go with something similar to a Weber Sweet Pea, but I changed my mind when I realized I would have to drill straight through my newly minted body.
I went through a few more designs and finally came up with something I could commit to. It consists of four brass pins which are about 1 and 1/4 inches long and have a slot to hold the string in place. These are positioned in holes drilled directly into the tail block. It was very simple to make and works well, although the pins could use a cover to protect the player's arm.