In this paper, we study the property of hereditary completeness of vector systems {xk}k=1∞ in a Hilbert space. A criterion of hereditary completeness is obtained in terms of projectors on closed linear spans of systems of the form {xk}k∈N, N⊂N. Developed technique has been used to prove that mixed systems of a hereditarily complete system are also hereditarily complete. In conclusion, the problem of possible defects in a nonhereditarily complete system is considered. © Tusi Mathematical Research Group (TMRG) 2025.