Join and separate objects in Blender: Ctrl+J and P

Assembling the parts of a piece of furniture into a single object, detaching a handle to give it another material, breaking an imported model into pieces: joining and separating objects are among the basic moves in Blender. This guide covers both operations, what they keep and what they lose, and the mistakes that often catch people out.

Joining objects: Ctrl+J

1. Switch to Object Mode.
2. Select the objects to join, then, last, the one that should receive everything: it becomes the active object, with a lighter outline.
3. Press Ctrl+J, or use the Object, Join menu.

All the selected objects are then merged into the active object. The result carries its name, origin and position, and appears as a single object in the Outliner (entry in French).

What joining keeps and what it loses

If an object carries a modifier whose effect you want to keep (a mirror, a bevel, a subdivision), apply it before joining, with Ctrl+A over the modifier or from its menu. Also apply the scale of the objects if they differ, to avoid unexpected distortion later.

Separating an object: the P key

Separating is done in Edit Mode (Tab). Press P, or go through the Mesh, Separate menu, and choose:

• Selection: the selected geometry is detached from its surroundings and becomes a new object. This is the most common option.
• By Material: one object per material, with the faces that use it. It applies to the whole mesh, not just the selection.
• By Loose Parts: one object per piece that is not connected to the rest. It also applies to the whole mesh, and is widely used on imported models delivered as a single block.

To quickly select a whole piece before separating it, hover over it and press L, or select part of it then press Ctrl+L to extend the selection to everything connected to it. The new object keeps the origin of the original object: re-centre it afterwards if needed (Object, Set Origin).

Joining is not welding

After Ctrl+J, the meshes are in the same object, but they stay independent: vertices that touch are not connected, and the parts can pass through each other. To go further:

• Weld overlapping vertices: in Edit Mode, Mesh, Clean Up, Merge by Distance, or M then By Distance, joins the vertices that sit in the same place.
• Connect two borders: select two edge loops and use Bridge Edge Loops, or fill a face between vertices with F.
• Get a single outer surface: the union of the Boolean modifier (entry in French) calculates the shared envelope and removes the internal parts, which matters for 3D printing.

When to join, when to keep separate

Joining simplifies a busy scene and groups the parts of one object. But a separate object keeps its own modifiers, can be hidden or animated independently, and is easy to replace. Moving parts (doors, wheels, jointed arms) usually stay separate, or are linked by parenting rather than joined. To organise without joining, Outliner collections are often enough.

Common mistakes

• The wrong object receives everything: it is the active object, selected last, that keeps its name and modifiers. Undo and start again in the right order.
• Ctrl+J does nothing: an object of a different type is selected (a curve with meshes, a camera), or the active object is not selected.
• P offers nothing: you are in Object Mode; separating requires Edit Mode.
• Black faces after joining: flipped normals on one of the objects; recalculate them in Edit Mode. The entry on the mesh (in French) explains normals.

The keys in this guide are listed with the others in the Blender shortcuts guide.