A PVC pipe fitting with four openings used at the top corner of a trellis frame, allowing two horizontal crossbars plus a vertical extension for a later scrog screen.
Jeremy explains you can buy 3-way corners but the 4-way is preferred when running a Grassroots trellis kit because the fourth opening leaves a hole to run a vertical PVC piece later, letting you build a scrog screen at whatever canopy height you decide on.
Discussed in: Ep 006