Docking Objects Together

6 min readUpdated Aug 2026

A lid that has to sit on a box, a pin that has to land in its hole: you can position those by hand and hope nobody edits the box. Docking does it properly. An object carries anchors, another object carries matching ones, and when you place the first near the second it snaps into the mate and stays there when the box moves.

What has to match

Docking is not proximity. Two anchors mate only when their pairing tags are the same string and their roles are opposite: a container offers a seat, a component takes one. A box carries containers, a lid carries components, and a single object can carry both. The tag rule is what stops a lid docking into a cable channel.

An object can also insist on being docked. One marked Required link has no legal position at all until one of its components is seated on a compatible container.

Drawing anchors, naming their tags and turning Required link on are the author's half of this, done in the object's own Anchors window. What an Object Exposes covers that half. This page is about what happens when you place the result.

Placing something that docks

Drag the ghost as usual. The magnet engages in two different ways, and the difference explains why docking feels long-range over one body and short-range everywhere else.

Hover a body that carries a compatible container and all of that body's anchors come into play, at any distance. You do not have to be near the seat, only over the body. Away from such a body, the magnet falls back to plain proximity: it catches what is close to the pointer.

When a pair engages, the two planes glue together with their normals opposed, the way two mating faces actually meet. Matched pairs draw as link lines so you can see what caught.

How much freedom is left depends on how many pairs matched. One pair fixes the position and leaves the spin free, so the rotation ring is still there and you can turn the object about the mate. Two pairs determine the pose completely, and the ring disappears. That is not a failure; there is nothing left to turn.

Press Tab to cycle candidate docks. On a body with several compatible seats, the first snap is rarely the one you meant.

Positioning dimensions hide themselves while the magnet is engaged. The dock is deciding the position at that moment, so there is nothing for them to drive.

A dock is a live relationship

When one object docks to another, the mate itself is recorded, not only the resulting position. The component's pose is worked out again from the container's frame on every rebuild. Move the box, edit the feature that made it, change a parameter that shifts the seat: the lid follows.

This is the reason to draw anchors at all. A hand-positioned object holds the coordinates it was given, and the first edit to the host leaves it floating in the wrong place.

Required link: the red ghost that will not commit

An object with Required link turned on has no legal position until it has docked at least once. Until then the ghost floats red and clicking does nothing at all. The placement session stays open until the object snaps to a compatible container or you press Escape.

There is no message and no toast to explain the dead click: the red ghost is the only signal you get. If you are stabbing at the viewport and getting no feature, that is what is happening.

Once it has docked once, the rule relaxes: dragging back out into empty space holds the last docked frame rather than going red again.

When it fails

Nothing snaps. You cannot go and read the other object's anchors from here. The Anchors window lives in the smart-object editor, and for a store object you have no access to the source at all, so work from what the viewport will tell you. Turn on SmartObject Containers in the 3D view window: if the host draws no seats, it carries no containers and no amount of dragging will produce one. If it draws seats, hover the body and press Tab through the candidates, because a pair can be engaging on a seat you were not looking at. Seats on screen, none of them offered by Tab, means the tags do not match or the roles are the same, and both of those are fixed in the object rather than in your document.

The magnet ignores a body you are sure has a seat. The long-range capture only happens over a body carrying a compatible container, which means role and tag both. Off that body you are back to proximity, so bring the pointer close to the anchor itself.

Clicking places nothing and the ghost is red. Required link is on and no dock has been made. Move over the container until the ghost turns normal, or press Escape and reconsider whether this object belongs on this part.

The rotation ring vanished. Two pairs matched, so the pose is fully determined. Drag the object off the dock if you need to turn it freely.

The dock caught the wrong seat. Press Tab to cycle to the next candidate rather than nudging the pointer around.