maviskw

These placemats do not have a Y-seam, but there is partial seaming. Peckish has the picture of what you discribe. It is called a partial seam, and I love doing them because of how the finished product looks. They are really easy once you get the hang of it. You have to add the new pieces "backwards". That is, you don't proceed going around the piece as you might normally do. You have to "back up" to the previous side to find a straight line onto which to sew the next piece.
When I sew the last piece on, the first piece is in my way. I fold it down, right sides together, then fold it back onto itself. Then the last corner is visible and easy to line up with the last piece.