Stained glass was a major art form in the medieval era. It was primarily used in cathedrals and other religious structures, often with a didactic purpose of illustrating stories from the Bible or the lives of the saints for a largely illiterate populace. Stained glass windows can be a great way to add a splash of color to your MOCs, and there are several ways to achieve this effect. When you are photographing interiors, putting a strong light source outside the stained glass can lead to great visuals.
In 2000, a printed stained glass window element appeared in King Leo's Castle and in the Royal Joust. This can be incorporated into MOCs, such as Obi-Juan-Kenobi's Ein Gedi Fortress.
Of course, you can get a lot more variety if you build your own windows using transparent bricks, plates, slopes and tiles. The most straightforward way to do this is to stack bricks and plates studs-on-top. You see this in official versions, like set 4729, Dumbledore's Office, or in MOCs like my Paul Revere's Ride. Here I have a wall two studs thick - the back wall is a stack of trans bricks in varied colors, and the front wall has arched window openings, to give some shape to the windows. I didn't take any pictures from the back, as this MOC was meant to be only seen from one side, but if you did want to make this attractive from both sides you would need a three stud thick wall.
That's pretty simplistic, though. You can get much subtler patterns if you use plates, or mix plates and bricks. Shmails shows an example of this in his flying buttresses entry to the CCC, discussed in this thread.
Here's another example from Plums_Deify's St. Anthony's Cathedral, discussed in this thread.
Transparent round plates will give an interesting textured look as in the Church of St Govan by Derfel Cadarn discussed in this thread.
Brendan Powell Smith mixed round plates and bricks in this window from his Church of Evil.
Turning your transparent plates by 90 degrees to be studs-sideways will give a more vertical feel to the window, which might be more appropriate aesthetically. Cathedrals were traditionally built with tall thin windows to direct the viewers' eyes upwards, towards heaven. In this window from Jojo's Canterbury Murder, the window and surrounding wall are constructed as a 1-stud thick sheet, that is turned on its side and slipped in a 1-stud-wide slot behind an opening in a studs-up wall. When you look from the other side you can see that the studs-up wall is built up on either side to mask the studs-sideways section. Notice that Jojo also includes gray plates, representing thin strips of masonry called tracery. Tracery will be discussed in more depth below.
You can get greater complexity to your patterns if portions of your windows have studs pointing in different directions from eachother. Take, for instance, Cyndi Bradham's Blue Glass Castle. In addition to the main rose window in the front, there are two other complex stained glass windows as well.
Let's take a more in-depth look at that main window, though. If you look closely, you'll see that there are four sections, with the studs pointing up, down, left and right. I've disected the window with red lines so you can easily see where these sections break down. The studs are all pointed outside of the frame, so where the sections come together it is either the bottoms or the edges of bricks that are touching, so they fit together flush, held in place by friction and because it is all sandwiched between arch pieces in front and behind. You could do this more simply by just dividing the circle into quarters, but Cyndi's is more complex so she could achieve her spiral pattern.
Romas shows another example where he has sections facing in different directions for the rose window of his Cathedral of St Macario. You might be able to see the construction a little more clearly in Tony Sava's interpretation of Romas' design. basically there's a stripe through the middle that is studs up (with a line of tiles along the top of the stripe), and the top and bottom portions of the window have the studs pointing to the left.
Daan Bargerbos realized you can get an interesting effect by fitting the studs of trans plates into the holes of pieces with lattices, such as the 1x4x2 fence, the 1x4x1 fence and the 8x8 grill plate, to come up with a number of window designs.
I tried my own hand at this and came up with these. Notice how you can get different looks by using either round or square 1x1 plates.
You get a different look from the other side, the side of the grill pieces, as you can see in this castle by Teddy.
MrTS put 2x2 plates on one side of the lattice and 1x1's on the other, to come up with a great window design for his battle scene.
You could use other elements to hold the transparent plates in place. Jens found that you could use the 2x2 turntable base for this effect in his throne room and kitchen. Notice that he used 1x2 plates for the transparent parts, as these are actually holding the turntable bases together. So in this case the stained glass is holding the tracery in place, rather than the other way around. Stone Goblin has an exploded view of this technique, so you can see exactly how this is done.
Stone Goblin also used this technique for his Catacomb Church. I couldn't resist including this, because in addition to the more narrow windows using the turntables, the studs-up main window is so beautiful, and I'd already included two great examples of studs-up windows above when I discussed that technique.
Teherean did something similar using technic bricks to hold the transparent plates to make small windows. This could also give an interesting effect if you turned it around.
A clear transparent 16x16 baseplate was released in 2007 in a mosaic set, and this can be the basis for studs-forward windows. BreadMan shows how this can be done for his Cathedral windows. The windows are built on the baseplate out of trans elements, along with gray tiles to frame them, and this whole section is then slipped into a waiting slot. Note that he uses 1x1 plates, tiles and slopes to get some texture variation, which also leads to slight differences in the transmission of light. 1x1 round plates would also give some interesting patterns, because there would be little bits of trans clear peeking around the edges. He discusses this MOC in this thread. Given the size of the 16x16 backing plate, this technique could be used to create truly huge stained glass windows, as in a full scale cathedral.
The release of 1x1x2/3 slopes (cheese) in transparent colors allowed for even more complexity in building stained glass windows. Here we see examples from Jim's church. There is a short article where he describes exactly how these were done.
Bruceywan showed that you can also turn the slopes by 90 degrees relative to eachother to get even more complexity in this cathedral window. Another builder, Sandy_Cash has also explored possible patterns using these slopes. While he was not using transparent slopes to form windows, you could easily apply his patterns for this purpose.
Katie Walker has done quite a bit of work finding how LEGO can fit together in all sorts of patterns for mosaics. Among other techniques she has found that cheese slopes can be put together to make complex geometric patterns and also to make pictures. She wrote up a tutorial on how to do this.
Others have also extended these designs using cheese slopes, such as the windows in Giles Gaer's Church of the Trinity.
Erik Amzallag came up with another great design held together by friction, his SNIR Window. He describes exactly how it's done in his announcement.
Don't forget the tracery. This is a web of thin pieces of stone or iron bars that holds the pieces of glass together. Don Bruce uses bars, antennae and other elements to make a lot of interesting examples of tracery for a Hogwarts MOC.
Milán Bikics incorporates plates and the 2x2 turntable base as tracery for the stained glass in his St Mirton Cathedral.
Victor designed a rose window in LDD, incorporating tracery directly into the 'glass'. He discusses it in this thread, with more pictures to help in construction.
I can't recall seeing this technique ever used for a stained glass window, but Brendan Powell Smith showed that you can combine headlight bricks to make a variety of complex patterns. He's used this a lot for floors, but these bricks are available in at least a few transparent colors, so I'd love to see them used for this purpose as well. He put together a tutorial on assembling these patterns. Hopefully in the future we'll see headlight bricks available in more trans colors.
It's not a fig-scale creation, but I really can't leave this topic without pointing to Brian Korte's great reproduction of the Bernini window made in 1666 found above the main altar in Saint Peter's Basilica.
Alyska Bailey Peterson has also made some amazing full-sized LEGO stained glass windows, such as Day Five from her Creation series.
For more stained glass, you should also check out the Flickr LEGO Stained Glass group.
Before leaving this topic, I should note that all of the techniques discussed above could be used with non-transparent colors to build mosaics, tapestries, rugs and floors.
Mosaic by me:
Tapestry by Bluesecrets:
Rug by me:
Floor by Brendan Powell Smith:
Floor by Katie Walker: