Two opposite ways contaminated water can flow the wrong direction back into the potable supply — and why the difference decides which backflow preventer actually protects you.
Backflow is the general name for the problem: non-potable or contaminated water flowing backwardinto the clean drinking-water supply, instead of the normal forward direction from the main to the fixture. It's a serious public health hazard — a single unprotected cross-connection can pull chemicals, boiler additives, irrigation runoff, or sewage back into a system meant to be safe to drink from — which is exactly why backflow prevention devices and assemblies are required by code at every cross-connection. But "backflow" isn't one mechanism. It happens two genuinely opposite ways, and confusing them is one of the most consequential mistakes in cross-connection control.