In the 1920s and '30s, the use of glass block was more or less limited to the side of the front door and in bathrooms. We still use glass block in those places, but today they are used almost everywhere else, too.

Since the center of the block is hollow, it provides dead-air space, which is good for insulation. Also, depending on the shape of the glass, light coming through it is diverted and distributed in odd patterns, giving the deflected light a decorative effect in the room. Some blocks are clear while others are more opaque, allowing light coming through to vary according to the style of block.

With typical blocks about 4 inches square, their uses are almost limitless. And while that size is typical, squares come in larger sizes as well.

Installation isn't difficult. The blocks have a rough coating around the edges that acts as a bond between the block and the mortar. One part Portland cement with one part lime and three parts sand will make a great mortar mix.

Expansion and reinforcing strips can be used to increase strength and flexibility. For the do-it-yourselfer, all these products are available at the home improvement stores.

Rosemary Sadez Friedmann, an interior designer in Naples, Fla., is author of "Mystery of Color."