the structure of "TCH + SACCH" is depend on timeslot?

Hi, I am studying the TCH channel, and read the BB source. I find that the structure of "TCH + SACCH" is depend on ts.that is when the timeslot is even, the 12 of a TCH multiframes is SACCH,and the 25 is idle;and When the timeslot is odd, the 25 of a TCH multiframes is SACCH,and the12 is idle. is that right? if that`s right, why ? I can`t find any Material about it. Thanks!