light-and-shadow effect. Kahn created additional visual interest by placing the twelve sets of piers in a pattern that is not quite equally spaced; the walls between the piers alternate between being slightly wider than expected and slightly narrower than expected. A large T-shaped window is located between each pair of adjacent piers. The horizontal part of each "T" fits in the space formed by the beams that support the roof. That is, the top bar of each "T" begins at the roof structure, as each beam does, extends downward as far as the bottom of the beams, and then extends horizontally across the entire space between two beams. As a result, the top parts of the T-shaped windows form a horizontal strip of glass just beneath the roof that extends the entire length of the building, interrupted only by the ends of the beams atop the piers. The vertical part of each "T" extends to the bottom of the ground floor. On the shorter sides of the building are six windows that could be described as distorted Ts; each has the form a square with a stubby tail descending from it. This photo shows the front of the building, the T-shaped windows, the ends of the roof-supporting beams on top of the piers, and one of the square windows beneath the beam at the end of the building. It is from the William S. Huff collection of photos listed below under External Links. Kahn used the T-shaped windows, sometimes called keyhole windows, to maximize the amount of natural light within the building while minimizing glare at desk height. Light enters the building mostly through the upper parts of the windows at the ceiling, brightening the rooms indirectly. The narrow, lower parts of the windows permit employees to see outside without letting in too much direct sunlight. Kahn used variations of the keyhole window in some of his subsequent designs. The use of this type of window spread and "soon became a cliché in the hands of other architects". The interplay of T-shaped window and structure is an architectural highlight of Kahn's design: the structure, which is exposed, determines the placement of the windows, and the windows in turn reveal and illuminate the structure, especially the beams running across the ceiling. Robert McCarter, author of Louis I. Kahn, which is the most extensive source of information about this building, provides this