There are several switch mode that can be realized using PIN Diodes. First of all, a Single Pole, Single Throw, also called SPST switch. The basic configurations are series and shunt. A symbolic diagram is given on the upper right hand side. Second, a Single Pole, Double Throw, also called SPDT switch. The basic configurations are series, shunt, series/shunt and combination of these three, are used for switch matrices. A symbolic diagram for SPDT mode is given on the lower right hand side.