This John Guest "Tee" (AKA: Angle Stop Valve) connects your water ionizer's [or filter system's] 1/4 inch hosing directly to the plumbing. It's just the right size to install between the standard 3/8 inch cold water shut-off and the cold water faucet feed line.
We prefer this Tee over the other tees out there, because it is easier to work with and it's just the right size.
It also has a valve that can allow the water to your filter system to be regulated for on/off and flow-rate.
See our post about: Installing a Tee