From RockGear:

The RockBoard Flat Y-Splitter cable is extra flat and space-saving. Designed to optimize pedalboard real estate. It features a slim rectangular body and extra thin angled plugs, to keep your setup tidy and neat looking. Do you have a TRS effects loop on your amp or effects pedals? The RockBoard Flat Y-Splitter Cable lets you insert any effects you like and does not take up a lot of space. Do your effects pedals allow stereo operation? Perfect, open up their possibilities with this simple cable.
 

• flat Y-splitter cable with rectangular profile
• for splitting TRS stereo signals into TS mono signals or as insert cable for TRS effects loops
• compact and super flat angled plugs
• flexible copper conductor 20 x 0.12 mm
• twisted copper shielding 34 x 0.12 mm
• PE (polyethylene) signal insulation
• outer PVC insulation
• capacitance of less than 10 pF/decimeters
• conductor resistance of less than 10 mOhm/decimeters
• 1 x angled 6.3 mm TRS audio plug
• 2 x angled 6.3 mm TS audio plugs (gray / black)
• tip = gray, ring = black
• operating temperature: -15º C to +70° C
• outer dimensions cable (W x H): 7 x 3 mm (rectangular)
• outer dimensions plugs (W x H): 12.5 x 7.5 mm
• black
• length 30 cm (11 13/16")