Centre Tapped Transformer Full Wave Rectifier Output Voltage
Suppose there are 10 turns on the primary and 10 turns on the secondary (5, then the center tap, then 5 more). In that case the top and bottom of the secondary relative to the grounded center is half of the input. Only one of the taps is ever conducting into the load at one time giving a waveform like this.

There are probably more than 10 turns these are presented for relative interpretation only.