Why are elbow joints 87.5 degrees instead of 90. All aspects of plumbing questions and answers, help, tips and information

[rhino.fog]

Hi
I have been installing some drainage and noticed that with the 110mm pipe the elbow joins are quoted as 87.5 degrees and not 90 degrees.

Does anyone know why?

The 45 degree junctions are quoted at 45 and the ones half that 22.5 so that makes sense but was wondering where the 87.5 came from.

[stoneyboy]

Hi rhino.fog
If you have a vertical pipe and want to put a "90" bend at the bottom into a horizontal pipe, if the bend was 90 degrees then there would be no fall on the horizontal pipe.
Regards S