There are two standard ways to describe a subspace, explicitly by giving a basis, or implicitly, by the solution space of the set of homogeneous linear equations. Therefore, there are two ways of describing a linear code, explicitly, as we have seen in the previous sequence, by a generator matrix, or implicitly, by the null space of a matrix. This is what we will see in this sequence. This leads to the following definition: H is a parity check matrix of a linear code, if the code is the null space of H. In this way, any linear code is completely specified by a parity check matrix. Suppose that we have a message of 4 bits, then we put them in the middle of the Venn Diagram, and we complete the empty three areas according to the following rules. The number of ones in every circle is even. This gives us three redundant bits