Weld

An object used to hold two objects together in a relative position, regardless of whether they're touching. This object is placed inside of a BasePart and the Part1 property determines which other part should be welded to the original part. Two CFrames, C0 and C1, then determine how the parts should be placed.

See also RigidConstraint for a newer alternative using the constraints system that does not require C0 or C1, but instead uses Attachments to control offsets through Constraint.Attachment0 and Constraint.Attachment1.

While the weld is Active, it maintains the part positions such that:

part1.CFrame * C1 == Part0.CFrame * C0

Root Part

Every assembly has a root part (see BasePart:GetRootPart()). When a weld's C0/C1 is modified, the root part will stay where it was.

Directionality

Welds do not have any directionality. Imagine rigid joints forming a tree, branching up from the root part such that all the parts up the tree from the root will move and their welded "children" will move with them.