The part of the sundial that casts a shadow used to tell time is called a "gnomon". Most stained glass sundials use the rod gnomons instead of the triangular sheet gnomons because they cast a smaller shadow on the stained glass and don't affect the light and colors passing through the glass as much. They're also easier to make and install. Another type of gnomon is a post gnomon. It usually meets the dial face at a 90 degree angle and may have a small sphere or a point at the end. You read the time where the shadow of the post's tip falls on the dial hour lines. The shadow cast by the end of a post gnomon or the pointed tip of a triangular sheet gnomon can also be used to tell the date on some sundials. The part of the gnomon that casts a date shadow is called a "nodus".
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For those who are adventurous, other unusual types of gnomons are possible. The sky's the limit! The left photo in the image shows a horizontal skylight sundial. the gnomon is simply a gap in a shade cloth placed above the sundial. Note how it uses a bright slash of sunlight to mark the hour. The center photo shows an aperture nodus in a disk gnomon where a sun beam shines through the hole onto the dial face. The right photo shows a dial hanging in front of a window's interior face. There is a painted dot gnomon/nodus on the window that casts a shadow on the dial face.
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