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Math o bolygon yw dihedron wedi'i wneud o ddau wyneb polygon sy'n rhannu'r un set o ymylon. Mewn gofod Euclidean tri dimensiwn, caiff ei ddirywio (degenerate) os yw ei wynebau yn wastad; ond oddi mewn i ofod sfferig tri dimensiwn, gellir ystyried dihedron gydag wynebau gwastad fel lens. Hen enwau arno yw: 'bihedra' a 'polyhedra fflat'.[1]
Un dihedron rheolaidd yw hwnnw a ffurfiwyd gan ddau bolygon rheolaidd; gellir eu disgrifio gan y symbol Schläfli {n,2}.[2]
| Diagram | 
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| Symbol Schläfli | {2,2} | {3,2} | {4,2} | {5,2} | {6,2}... |
|---|---|---|---|---|---|
| Coxeter |   
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| Wynebau | 2 {2} | 2 {3} | 2 {4} | 2 {5} | 2 {6} |
| Ymylon a chorneli |
2 | 3 | 4 | 5 | 6 |
- ↑ Kántor, S. (2003), "On the volume of unbounded polyhedra in the hyperbolic space", Beiträge zur Algebra und Geometrie 44 (1): 145–154, MR 1990989, http://www.emis.ams.org/journals/BAG/vol.44/no.1/b44h1kan.pdf, adalwyd 2018-09-20.
- ↑ Coxeter, H. S. M., Polytopau Rheolaidd (3ydd ed.), Dover Publications Inc., p. 12, ISBN 0-486-61480-8