A modular block system comprises a first block, a second block, a third block, a fourth block, and a capstone, the first block having a front face, a back face, a first side, a second side, a top, and a bottom, with the front face being wider than the back face and the first side being slanted from the front face toward the back face, the second block having a front face, a back face, a first side, a second side, a top, and a bottom, with the front face being wider than the back face and the first and second sides being slanted from the front face toward the back face, the third block having a front face, a back face, a first side, a second side, a top, and a bottom, with the front face being wider than the back face and the first side being slanted from the front face toward the back face, and the fourth block having a front face, a back face, a first side, a second side, a top, and a bottom, with the front face being wider than the back face and the first side and the second side being from the front face toward the back face.