Abstract: An adjustable modular building unit is used to construct a truss structure of any given size and shape which is joined with other truss structures to form a building. Before adjustment, the modular building unit has a six-sided rectangular parallelepiped shape. However, the bottom of the modular building unit and a diagonal brace member are adjustable in length so that the vertical, lengthwise cross section of the modular building unit is adjustable to various isosceles trapezoidal configurations. Preferably, the modular building unit is constructed of various chord members so that each side forming the trapezoidal configuration is rotatably connected to the adjacent side and the diagonal brace member provides stability. In order to construct the truss, each building unit is releasably attached to an adjacent building unit which has been appropriately adjusted. A base unit for each end of the truss is also provided which includes a rotatably side supporting plate.