Transformer secondary C.T. Label
Center-tapped. In this case it means that the secondary winding has two opposite phases and a middle connection.

Such a transformer looks like this:







simulate this circuit &ndash; Schematic created using CircuitLab

For a 5V AC secondary transformer, the voltages are:


V1 to V2: 5V AC
V1 to Vct: 2.5V AC
V2 to Vct: 2.5V AC