Hi, I know what you mean and it is called keystoning. If you go to the bottom of this page, there is a search, type in keystoning, and you will find something posted by flagtruck and a link to Sonia King. It is an excellent tutorial. I'd post the link here but I don't know how!? Also if you google keystoning for mosaic you will find more info. I've been practising keystone but it isn't as easy as it looks.
Remember to use the search tool at the bottom, it will bring up some wonderful old posts, cool things I'd forgotten about, some pics from photobucket seem to be deleted but there are still some good pics there. You can also type in a member name if you want to see something specific to that person.

Here you go glass additiction, this process is called
"keyholing". If you will notice, not every tile is cut, just what it takes to fit the circle. Hope this helps, let me know if you have more questions. Carol

Here is a link that might be useful: gourd birdhouses

thanks squirrelly for your help I was able to find what I was looking for.