We have plunged it and snaked it, but it seems that when we drain the tub upstairs it overflows the downstairs toilet, when we do laundry which is downstairs, it overflows the toilet. Dare we try the dishwasher. Do all the drains lead to one toilet? Also could the drain vents be stopped up? It is an older home. Any suggestions are welcome. Thx. Charlotte

No all drains do not lead to one toilet but they are like branches on atre all leading to a trunk. The trunk is your main drain line and only one line exits to home toward the city sewer or septic. You clog is after the basement toilet so you could start there with a 1'2" snake, pull the toilet if you can't find your clean out on the main. If the cleanout has standing water the clog is between you and the outside of the house. If the clean out is dry then the clog is upstream in he drain line.