Related Clues:

• 1 + 2 + 3, e.g.
• Measure of length
• Pipe fitter's union?
• Architectural annex
• Architectural wing, perhaps
• Connecting pipe
• Letter after kay
• Pipe fitter's joint
• Simple math problem
• Right-angled wing
• Wing that can't flap
• It may be tidy
• A kind of add on
• Corner shape
• 90 degree pipe joint