Abstract: A modular wall or fence construction system includes at least first and second I-profile blocks, each having an I-shape in planform. The I-shape defines a parallel pair of flanges across opposite ends of a web extending between the flanges. The I-profile blocks have generally planar and parallel upper and lower surfaces and a uniform vertical height. Each of the flanges of the first and second I-profile blocks have radiused rib keys formed in opposite parallel edges of the flanges. The edges are distal on the flanges relative to the web. A first pair of the radiused rib keys are a pair of convex rails along the parallel edges on a first side of the web. A second pair of the radiused rib keys are a pair of concave channels along the parallel edges on a second side of the web opposite the first side of the web. Each flange has oppositely disposed, on opposite edges, one convex rail and one concave channel.