I was typing my comment on the way out the door from work, and I thought to amend my comments to remind you that USUALLY a center tapped transformer precedes a full-wave rectifier configuration, which has one polarity for its output. Dual polarity from a center tap requires a bridge circuit.
I read a post on another thread that implied you are a student. I didn't go into detail about why a 24 volt transformer isn't appropriate for both a -10 and +15 volt supply, so in case you haven't learned this yet I will describe it.
Obviously, you need a center tap in a linear power supply to achieve both polarities. remember that in full wave and bridge rectification, the negative pulse gets "flipped" to fill the inbetweens of the positive pulses, then smoothed, filtered, and regulated.
Well, in the dual polarity supply, the mirror image is happening on both sides of the center tap. Thus, the full 24VAC on your secondary is cut in half for each polarity.
The best you can hope for is 12 volts of unregulated, pulsed DC.
But this is unacceptable to drive most devices, so we have to smooth out and regulate the voltage.
The pulsed DC is PEAK 12 volts, but it is sinusoidal. I don't care about doing the math, but you can figure on losing a couple of volts when your regulator takes over. It needs something called HEADROOM to operate properly, or else it really isn't regulating.
So with our 24 volt center tap design, we're doing well to get a -10 and +10 supply out of it with efficient regulation and optimal smoothing.