Joints

A joint, called a "connection" while a game is loading, is something that connects two parts together. Common types of joints are Welds, Snaps, and Hinges. Joints are represented using a JointInstance object, which has four important properties: C0, C1, Part0, and Part1.

Advanced

Fixed Joints

Joints have a conceptual origin coordinate frame, to which both parts are attached.

jointOrigin == joint.Part0.CFrame * joint.C0 == joint.Part1.CFrame * joint.C1

In the case of fixed joints, this origin position is not important.

It is sufficient to make the origin position the center of Part1, and C1 the identity CFrame. For example:

local cf = ... --The transformation from A to B

local weld = Instance.new("Weld")
      weld.Part0 = workspace.A
      weld.Part1 = workspace.B
      weld.C0 = cf
      weld.C1 = CFrame.new(0,0,0)

Will ensure that workspace.A * cf == workspace.B

Rotating Joints

In a rotating joint, the conceptual origin is important. This dictates the position, and in some cases, the axis around which the joint rotates.