PrimaryPart

Property of: Model

Value Type: BasePart

Description:

Points to the primary part of the Model. The primary part is the BasePart that acts as the physical reference for the pivot of the model. That is, when parts within the model are moved due to physical simulation or other means, the pivot will move in sync with the primary part. If the primary part is not set, the pivot will remain at the same location in world space even if parts within the model are moved.

Note that when setting this property, it must be a BasePart that is a descendant of the model. If you try to set Model/PrimaryPart to a BasePart that is not a descendant of the model, it will be set to that part but reset to nil during the next simulation step — this is legacy behavior to support scripts which assume they can temporarily set the primary part to a BasePart which isn’t a descendant of the model.

The general rule for models is that:

Models whose parts are joined together via physical joints such as WeldConstraint|WeldConstraints or Motor6D|Motor6Ds should have a primary part assigned. For example, Roblox character models have their Model/PrimaryPart set to the HumanoidRootPart by default.
Static (usually BasePart/Anchored|Anchored) models which stay in one place unless a script explicitly moves them don’t require a Model/PrimaryPart and tend not to benefit from having one set.

Code Samples

Throwing Dice

This code sample creates and throws a dice model, then outputs whether it landed with the blue side facing up. If Model/PrimaryPart was not set, the pivot / bounding box of the dice would rotate, as the parts inside of it are physically simulated during the roll.

			
				Throwing Dice

				Expected Output
			
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			-- Create a dice model with two halves and attach them together
local diceModel = Instance.new("Model")
diceModel.Name = "ChanceCube"
local diceTop = Instance.new("Part")
diceTop.Size = Vector3.new(4, 2, 4)
diceTop.Position = Vector3.new(0, 1, 0)
diceTop.Color = Color3.new(0, 0, 1)
diceTop.Parent = diceModel
local diceBottom = diceTop:Clone()
diceBottom.Position = Vector3.new(0, -1, 0)
diceBottom.Color = Color3.new(1, 0, 0)
diceBottom.Parent = diceModel
local weld = Instance.new("WeldConstraint")
weld.Parent = diceModel
weld.Part0 = diceTop
weld.Part1 = diceBottom

-- Put the dice up in the air above the workspace origin (does not require a primary part)
diceModel.Parent = workspace
diceModel:PivotTo(CFrame.new(0, 10, 0))

-- Assign the primary part before physical simulation
-- Without this line, the script will always output the same thing and the bounding box of the model will not change orientation
diceModel.PrimaryPart = diceTop
 
-- Wait a bit before rolling the dice (let it settle onto the floor)
for i = 5, 1, -1 do
	print("Rolling dice in...", i)
	task.wait(1)
end

-- Roll the dice
diceTop:ApplyAngularImpulse(Vector3.new(15000, 1000, 5000))
diceTop:ApplyImpulse(Vector3.new(0, 3000, 0))
task.wait(1)

-- Wait for the roll to complete
while diceTop.AssemblyLinearVelocity.Magnitude > 0.1 or diceTop.AssemblyAngularVelocity.Magnitude > 0.1 do
	task.wait()  -- Wait 1 frame
end

-- Get the dice orientation, impacted by the primary part
local orientation = diceModel:GetBoundingBox()
if orientation.YVector.Y > 0.5 then
	print("It's the boy!")
else
	print("It's his mother!")
end
		
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