The Difference Between GU4, GZ4, GU10, and GZ10 Lamp Bases

The codes GU4, GZ4, GU10, and GZ10 refer to the specific type of pin style connectors on the bottom of the bulb. Knowing the difference between these is key to determining which bulb is the correct one for your fitting. The types of fittings are primarily used in spotlights and downlights are available in halogen and LED technologies.

The letters in these codes indicate the base type and the number advises the distance between the two pins which is measured in millimeters.

Understanding the 'G' and 'U' Modifiers: The Bi-Pin Base

The letter 'G' refers to the general pin-type base and the letter 'U' means that the lamp base has a retaining feature molded into the base, which helps to lock the bulb into the socket when it is twisted. This helps to ensure the bulbs are safely and securely connected.

Comparing the GU and GZ Bases

The difference between GU and GZ lies in the shape of the base plate and how the pins fit into the fixture.

The Significance of 'Z' (GZ Base)

The 'Z' in GZ stands for Zylindrischer which is German for cylindrical. It is a non-retained base, which means it does not have the same locking groove as the GU type. A GZ base will have straight pins and a flat contact surface that sits flush against the socket.

Detailed Comparison of the Four Bases

1. GU4 vs. GZ4

Feature	GU4 Base	GZ4 Base
Voltage	Typically Low Voltage (12V)	Typically Low Voltage (12V)
Length between pins	4 mm	4 mm
Pin Type	Thin, straight pins	Thin, straight pins
Key Difference	The base or pin configuration allows for the retaining notch or feature.	Lacks the retaining feature of GU. Generally fits flush.
Common Use	MR11 Lamps, small spotlights, under-cabinet lighting.	MR11 Lamps, small spotlights.

GU4 and GZ4 bulbs are primarily used for 12V low-voltage lighting. The 4 in both part numbers refers to there being 4mm between the pins but it is important to make sure that you purchase the correct part choosing a GU4 if it is a GU4 you had previously for example.

2. GU10 vs. GZ10

Feature	GU10 Base	GZ10 Base
Voltage	Mains Voltage (220-240V)	Mains Voltage (220-240V)
Length between pins	10 mm	10 mm
Pin Type	Larger, thicker pins with a flat or slightly flared end. Bayonet-like push-and-twist fitting.	Similar size pins, but often strictly cylindrical with no bayonet flare.
Key Difference	Twist-and-Lock mechanism. The base has a wider flange that requires twisting to lock into the fixture for mains safety.	Push-in mechanism. The base is often strictly cylindrical and lacks the flange needed for the twist-and-lock feature.
Common Use	Standard downlights, track lighting.	Older or specialized mains-voltage fixtures. Less common than GU10.

The 10 with the description for GU10 refers to the pins being two millimeters apart. Much like the GU4 and GZ4 it is important to make sure that you do not purchase a GZ10 if you had a GU10 previously.

Created by Adonia Watt on 6th February, 2026

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Adonia Watt

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