Horizontal or Vertical 
One or more lines or two or more points. 
The lines become horizontal or vertical (as defined by the current sketch space). Points are aligned horizontally or vertically. 
Collinear 
Two or more lines. 
The items lie on the same infinite line. 
Coradial 
Two or more arcs. 
The items share the same centerpoint and radius. 
Perpendicular

Two lines. 
The two items are perpendicular to each other. 
Parallel

Two or more lines. A line and a plane (or a planar face) in a 3D sketch.

The items are parallel to each other. The line is parallel to the selected plane.

ParallelYZ 
A line and a plane (or a planar face) in a 3D sketch. 
The line is parallel to the YZ plane with respect to the selected plane. 
ParallelZX 
A line and a plane (or a planar face) in a 3D sketch. 
The line is parallel to the ZX plane with respect to the selected plane. 
AlongZ 
A line and a plane (or a planar face) in a 3D sketch. 
The line is normal to the face of the selected plane. 
Relations to the global axes are called AlongX, AlongY, and AlongZ. Relations that are local to a plane are called Horizontal, Vertical, and Normal.

Tangent

An arc, ellipse, or spline, and a line or arc. 
The two items remain tangent. 
Concentric

Two or more arcs, or a point and an arc. 
The arcs share the same centerpoint. 
Midpoint

Two lines or a point and a line. 
The point remains at the midpoint of the line. 
Intersection

Two lines and one point. 
The point remains at the intersection of the lines. 
Coincident

A point and a line, arc, or ellipse. 
The point lies on the line, arc, or ellipse. 
Equal 
Two or more lines or two or more arcs. 
The line lengths or radii remain equal. 
Equal Curvature 
Two splines. 
The radius of curvature and the vector (direction) matches between the two splines. 
Symmetric

A centerline and two points, lines, arcs, or ellipses. 
The items remain equidistant from the centerline, on a line perpendicular to the centerline. 
Fix 
Any entity. 
The entity’s size and location are fixed. However, the end points of a fixed line are free to move along the infinite line that underlies it. Also, the endpoints of an arc or elliptical segment are free to move along the underlying full circle or ellipse. 
Fix Slot 
A slot sketch entity. 
The entity’s size and location are fixed. 
Pierce 
A sketch point and an axis, edge, line, or spline. 
The sketch point is coincident to where the axis, edge, or curve pierces the sketch plane. The pierce relation is used in sweeps with guide curves. 
Merge Points 
Two sketch points or endpoints. 
The two points are merged into a single point. 
Doubled Distance 
A centerline and any sketch entity. 
The sketch entity is dimensioned at twice the distance from the centerline. 
Equal Slots 
Two or more slot sketch entities. 
The items have equal lengths and radii. 
On Edge 
Edges of a solid. 
The edges of the solid are projected to the sketch plane using the Convert Entities tool. 
On Plane 
Sketch entities on a plane. 
The sketch entities reside on the plane. 
On Surface 
Sketch entities on a surface. 
The sketch entities reside on the surface. 
Tangent to Face 
A sketch entity and a solid face. 
The sketch entity and face are made tangent to one another. 
Traction 

See Using Traction and Belts for Layout Sketches. 