The "F" Style 1/4" to 1/4" branch fittings are a convenient alternative to "Y" and "T" connectors. They are used to link two water-blocks in parallel using dissimilar tubing sizes. The F fittings are made of POM (Acetal Copolymer). The advantages of these barb fittings compared to Y's or T's are: Cleaner routing of the tubing, reduction from "large" tubing size such as 1/2" or 3/8" to devices installed in space constrained areas and necessarily requiring 1/4" ports & minimal pressure drop.