Rhino - Vocabulary — David Barbarash

Why this matters

CAD geometry and Rhino geometry are not the same thing. A polyline in CAD becomes a curve in Rhino. A closed polyline becomes the source of a surface. A surface is not a solid. A mesh is not a surface. Getting these terms wrong means you cannot describe what you're looking at, which means you cannot diagnose what's wrong, which means you cannot fix it. Learn the vocabulary before you encounter the problems.

Geometric types

Term	What it is
NURBS	Non-Uniform Rational B-Spline. The mathematical description of curves and surfaces in Rhino. More precise and more editable than mesh geometry for design modeling purposes.
Curve	A Rhino line or arc — equivalent to a CAD polyline. All CAD linework imports as curves.
Surface	A NURBS surface defined by a network of curves. Has no thickness — it is geometry with area but no volume.
Polysurface	Multiple surfaces joined along shared edges. When all edges are joined and the interior is enclosed, a polysurface becomes a closed solid.
Solid	A closed polysurface with no naked edges. Has a defined interior volume. Required for boolean operations.
Mesh	A faceted representation of geometry made of triangles or quads. Less mathematically precise than NURBS but faster to render. Terrain created with Grasshopper is a mesh before it becomes a NURBS surface via DRAPE.
Open object	A surface or polysurface with at least one exposed edge (a naked edge). Cannot be used as a solid for boolean operations.
Closed object	A surface or polysurface with no exposed edges. Encloses a volume.
Control points	The defining vertices of a NURBS curve or surface. Moving control points changes the shape of the geometry they define.
Isocurves	The grid lines visible on a NURBS surface showing its internal structure. Not separate objects — visual indicators of the surface shape.
Seam	The edge where a closed surface meets itself. Important for texture mapping and Boolean operations.
Naked edge	An edge of a surface or polysurface that is not shared with another surface. Indicates an open, non-watertight model condition. Surfaces with naked edges cannot be extruded or used in boolean operations reliably.

Interface terms

Term	What it is
Viewport	One of the four standard views: Perspective, Top, Front, Right. Each is a camera pointed at the same model from a different angle. You can create custom viewports to save specific views or locations.
CPlane (Construction Plane)	The active working plane — the flat reference grid that grounds all drawing and modeling operations. By default aligned to World XY (Top view). The equivalent is `UCS OB` in CAD; in Rhino it is achieved via the CPlane commands.
Gumball	The interactive transform widget that appears on selected objects. Drag the arrows to move, the arcs to rotate, the squares to scale. Faster than typed commands for quick adjustments. Single click on a gumball element for precise text entry.
World coordinates	The absolute XYZ coordinate system of the modeling space. X is right, Y is forward, Z is up.
Display mode	How geometry is shown in a viewport: Wireframe (lines only), Shaded (solid colors), Rendered (materials), Arctic (soft ambient, good for form reading), Ghosted (transparent shaded).
Properties panel	Shows selected object information: layer, material, object type, dimensions. The equivalent of the AutoCAD Properties palette.
Command line	As in AutoCAD — read it, respond to it. Rhino is command-line driven. Right-click repeats the last command when nothing is active.
Osnap bar	Persistent snap settings at the bottom of the screen. Same logic as AutoCAD: End, Mid, Int, Cen, Near, Perp, etc.

From the office

The difference between a surface and a solid becomes critical the moment you need to do a boolean operation or create a material that has thickness. A wall is a solid. A ground plane is a surface. A planting bed edge is a closed curve waiting to become a surface. Build your mental model of what type of geometry each element requires before you start modeling it — redoing geometry types mid-model wastes time.