A right-handed, differential trihedron.
Or, if you wish, Gauss' Fundamental Theorem of Normal Axonometry.
giggle.
This is problem #74 in Heinrich Dorrie's [i]100 Great Problems of Elementary Mathematics.[/i]
__________
These are my self study materials. Let me know how I can make them more useful to you.
[b]Unit Sphere[/b]
[list]
[*]Setup: [url]http://www.geogebratube.org/material/show/id/101282[/url]
[*][b]→ Trihedron:[/b]
[*]Base Object: [url]http://www.geogebratube.org/material/show/id/105255[/url]
[*] Spherical Coordinates {link}
[*] Meridian, (Horizon Points)
[*] Latitude, (Horizon Points)
