Modular system for precise construction where this system contains two basic blocks (3,5) and two derived blocks (4,6) with a rectangular block shape and identical height (b) where the first basic block with a square-shaped base has in one pair of the opposite side vertical faces always respectively one external groove and the second basic block with a rectangular-shaped base and two derived blocks (4,6) with a rectangular-shaped base also have in their shorter opposite vertical faces always respectively one external groove where these external grooves are made in the full height (b) of the above-mentioned block s (3,4,5,6) and they are created symmetrically around the symmetry plane protruding through the vertical axis of the opposite vertical faces, in which external grooves are located while for all blocks (3,4,5,6) applies that on the same cross-section at least one internal first width (e) of each external groove in plane parallel with the surface of the sides vertical face, in which it is formed, is greater than its second width (d) in the plane of the surface of this vertical face and where the derived blocks (4.6) have in their centre an internal groove for tight insertion of internal connecting bars for the purpose of creating a self-locking connection created symmetrically around the symmetry plane protruding through vertical axis of the shorter opposite vertical faces and corresponding by its shape and dimensions to the connection of two external grooves of any two blocks (3,4,5,6) from the set of basic blocks (3.5) and/or derived blocks (4,6), which are intended to be laid and bound side-by-side in a row characterized by the fact that the first basic block has a square-shaped base with a third width (a) and is equipped on one and/or the other side vertical face of the second pair of opposite side vertical faces with one external groove created in the full height (b) of the above-mentioned block symmetrically around the symmetry plane protruding through the vertical axis of the opposite vertical face s, in which external grooves are located, the first derived block is derived from the first basic block and has a rectangular-shaped base with the third width (a) and a length two times the third width (a) and the internal groove in the centre of its base is created along the full height (b) and this first derived block is equipped along the full height (b) with minimum two and maximum six external grooves where in each shorter side vertical face is symmetrically around the symmetry p plane protruding through the vertical axis of the opposite shorter vertical faces, in which external grooves are located is created maximum one external groove and in each of the longer vertical faces are maximum two external grooves created symmetrically around the symmetry planes parallel with the shorter vertical faces and located in the distance (a/2) half the length of the third width (a) from the shorter vertical faces, while these symmetry planes are distanced from each other at the third width (a), the second basic block has a rectangular-shaped base with the first length (a) beeing equal to the third width (a) and a forth width (c) shorter than the third width (a) and is equipped on one and/or the other longer vertical face with one external groove created along the full height (b) symmetrically around the symmetry plane protruding through the vertical axis of the opposite longer vertical face s, in which external grooves are located, and the second derived block derived from the second basic block has a rectangular-shaped base with the length two times the length of the third width (a) and a forth width (c) shorter than the third with (a) and the internal groove in the centre of its base is created along the full height (b) and this second derived block is equipped along the full height (b) with minimum two and maximum six external grooves where in each shorter vertical face symmetrically around the symmetry plane protruding through the vertical axis of the opposite shorter side vertical faces, in which external grooves are located is created maximum one external groove and in each of the longer side vertical faces are maximum two external grooves created symmetrically around the symmetry planes parallel with the shorter side vertical faces and located in the distance half the length of the third width (a) (a/2) from the shorter vertical faces, while these symmetry planes are distanced from each other at the third width (a), and further the system contains external connecting bars for insertion into the external grooves and internal connecting bars for insertion into the internal grooves side-by-side or one on top of the other laid blocks (3,4,5,6) where the blocks (3,4,5,6) are positioned mutually in such a way that the external grooves and the internal grooves of neighbouring blocks (3,4,5,6,) follow each other dimensionally in vertical symmetry planes dividing the second widths (d) of the external grooves cross-sections, or second widths (dd) respectively, of the internal grooves cross-sections into halves.