Before 1916, there was no standard pattern for fire lookout structures. The D-6 cupola cabin was the first; a wood frame 12x12' cab with a 1/4-size second story; windows all around. Its design grew out of a sketch drawn inside a tent atop Mount Hood in Oregon, and was given the name "D-6" from USFS District-6. About 100 were built between 1916 and 1929.

The "L" series began in the Northwest also. As early as 1920, Montana rangers were designing their own version of what a cupola cabin should look like. Made of hand-hewn logs cut on-site, some dove-tail cornered with great skill, the "D-1" evolved into the "L-2", "L-3", and "R3 Nezperce" cabin. 70+ were made.

"L-4" was by far the most popular live-in lookout. It came in three generations; with a 14x14' wood frame cab, windows all around; sitting on the ground, or atop pole or timber towers up to 100' tall. The 1929-1932 version featured a gable (2-sided) wood shingle roof. The 1933-1935 version had a 4-sided hip roof. The 1936-1952 version had a similar hip roof, with extended ceiling joists to hold the window shutters open. They were ordered in kits, packaged for mule or truck transport. More than two thousand were shipped from USFS warehouses in Missoula, Spokane, Portland, or fabricating mills in Vancouver, WA and Columbia Falls, MT.

"L-5" was similar to the 1933 L-4, but only 10x10'. "L-6" was also similar, but only 8x8'. "L-7" had a low gable roof, was 7x7', wood frame, and had only two window sets each side; with access by door from the catwalk, or through a trapdoor inside the cab.

"R-6" was developed in Oregon's USFS Region-6 in 1953. It has a flat roof, 15x15' frame live-in cab with 7 sets of windows each side. Several hundred exist today, from ground level, to 84' high on treated timber towers.

The most popular lookout structure elsewhere in the U.S. is the all metal tower. Its primary supplier, the Aermotor Co. (same company that made most of the windmills for farms all across America) offered its fire tower in a dozen different designs, since the early 1920s. Most common is the "MC-39"; a heavily galvanized free-standing batter-legged hurricane-proof structure with a 7x7' metal cab. It came in a wide choice of heights: 33' (4 flites, 44 steps); 45'9" (5 flites, 61 steps); 59'3" (79 steps); 79'6" (8 flites, 97 steps); 99'9" (9 flites,133 steps). Also a few dozen were 120' tall; and the tallest, 176', is still in service at the Woodworth State Forest near Alexandria, Louisiana.

"LS-40" was similar at a glance, but lighter in material and design. They were found in several styles; with straight ladders or criss-cross stairs; made by Aermotor and a half dozen other manufacturers across the U.S. during the 1930s.