The Pattern Breakdown

Note: This is not an exact pattern, more of an explanation of the plan of construction. Experienced knitters will get this certainly, and novices will roll their eyes. Mitered square tutorial available here.

Side panel update here.

   

When I made the prototype Diamond Panel Jacket, I did it in separate fronts and back, sewing up the shoulders, joining the side panels (3 needle bind off) and then knitting the sleeves last, in the round.

Later I realized that I could knit the jacket seamlessly, without slits. That sounded much better to me.  

To get there, one makes the front diamond panels and the back diamond panel first and then joins them at the shoulder with a quarter square triangle, picked up from the edges of the front and back panels.

To knit this shape, decreases are made at the center and both edges of the front side rows only. When only 5 sts remain, bind off. This produces a straight edge at the top of the shoulder, and nicely joins the front and back.

The diamonds and half diamonds are knit in a specific order, and of course if you are familiar with mitered squares or domino squares, you will recall that consecutive shapes are knit from stitches picked up from the previous shape.

Number 33 is that quarter diamond above.This is a diagram of the side panel, which is knit in the round. No side slits in this version. Stitches are picked up from the diamond panels, which means a little math is required. If the diamond is knit from 29 stitches, the half diamond is knit from 14, and since the edges of the panels are all (but the bottom triangles) half diamonds, pick up 14 stitches from each half diamond. The bottom triangles, 29 and 30 on the back panel and 17 and 18 on the front are 14 sts, with decreases at each edge on the front side only. So, after all the stitches are picked up from the front and back panel edges, some stitches need to be cast on at the bottom hem to complete the round. This is where you need to figure out what your stitch per inch is and decide how much more width is needed. Math again.

 The diagram shows the amount of stitches I used for my gauge, and the 3 represents the sts of the triple decrease or S1. kstog, psso. It will form miters at the bottom of that side panel as shown. The remaining 60-65 stitches will be the beginning of the sleeve.

To knit the sleeve, knit in the round on two circular needles, and decrease sts at the beginning and ending of the row, every six ridges of garter stitch.
See this post for side panel update.
NOTE: after making several of these jackets I changed the neckline a bit.
  The neckline was too small and high in the original version, so I extended it a bit by adding two more triangles, a half-triangle ( A)  and (B) a quarter triangle. Also notice that the back has changed too, ending in half triangles. The shoulders are the only seams.