Angle Specification

Ok this isn't a question about Solidworks, it relates to a tutorial in Sketchup... but I can't find the answer on the net or in Engineering
books, so I'm thinking that it might be US terminology.
In the Sketcup tutorials they use "8:12" to specify an angle? What does this mean? What is the angle in decimal?
I'm sure there is a very simple answer to this.
Cheers.
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will snipped-for-privacy@yahoo.co.uk says...

That terminology is common in construction, especially in roofing where "8:12" would indicate the slope as 8 units "rise" in 12 units "run". 8/12 is the inverse tangent of the angle.
Art W.
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