**Figure 2**
The conventional Harker construction for SAD phasing. The total structure factors for **F**^{+} and **F**^{−}* (the complex conjugate of **F**^{−}) are sums of complex numbers (which can be represented as vectors) with common components. In the Harker construction we represent **F**^{+} as the vector sum of the imaginary contribution from the anomalous scatterers (**F**_{H}^{+}′′), the real contribution from the anomalous scatterers (**F**_{H} + **F**_{H}′) and the unknown contribution from the rest of the protein (**F**_{P}, represented with two possibilities in solid and dashed arrows). Since the amplitude of the total structure factor, |*F*_{o}^{+}|, is known, the **F**_{P} vector must end up on the blue circle, which is centred on the tail of the **F**_{H}^{+}′′ vector and has a radius of |*F*_{o}^{+}|. Similarly, **F**^{−}* is represented as a vector sum, starting with its imaginary contribution from the anomalous scatterers (**F**_{H}^{−}′′) and then sharing the remaining real scattering components. The red circle, which is centred on the tail of the **F**_{H}^{−}′′ vector and with a radius of |*F*_{o}^{+}|, crosses the blue circle at the two possible values for **F**_{P}; the shorter of the two possible vectors is more probable. If the structure factor will be used for a map containing the anomalous scatterers, the origin of the Harker construction is taken at the base of the vector for the real contribution from the anomalous scatterers, indicated by a cross. |