I've been asked about my buttonholes a few times, mainly by people wanting to know how to do them. There's a good set of printed instructions here but I have a few things to add to it.
The type of cloth will dictate how narrow a bite you can take- loose cloth requires a wider bite, tight cloth you can get away with a narrower bite. It makes a difference in the appearance. Consider the buttonholes at the top of the post- they have a raised appearance like the Milanese buttonhole. The buttonholes below appear much flatter.
What is the difference? They were all made with the same size silk twist, the same gimp, using the same stitch type, and by the same person (me). To get the flat buttonhole, I take a fairly wide stitch so that the gimp sits under the leg of the stitch and the knot sits to the side of it. For the raised buttonhole, I take a much narrower stitch and pull straight up when making the knot in order to place the knot directly on top of the gimp. This gives the volume. I filmed the making of the grey sleeve shown above; I'm not sure if anything will be visible in the internet-friendly version, but let's give it a shot.
EDIT- Apparently some browsers only display half the video screen, so by clicking on the title of the video (Hand Made Buttonholes) you can go to the vimeo site where you should be able to see it complete. In the meantime I will try to figure this thing out..... still more technical problems. If you get jagged lines, try clicking through the full res version on Vimeo- I think that works better.
Those who are interested in the materials used can check out this blog post where I discussed the various threads http://tuttofattoamano.blogspot.com/2009/12/hand-made-buttonholes.html
Unfortunately, I don't know where to get the Agreman gimp online. Richard James Weldon stocks it in London, Lafayette Saltiel Drapiers stocks it in Paris and sells the black by the meter rather than by the spool, you could try calling Bergen Tailor Supply, B. Black or Ely Yawitz- they may stock it even if it's not on their websites. Next time I see my Gutermann rep I will ask if he knows of anybody who stocks it, but I doubt he will know.