The Ultimate Guide To All The Confusing 2 Pin / 2 Prong Light Bulb Fittings

Learn The Mystery Of The "G" Light Socket Bulbs

When shopping for replacement light bulbs, many people are puzzled by the wide range of two-pin (or two-prong) fittings that all look deceptively similar. You may have seen designations like G4, GU4, GU5.3, GY6.35, GU8, GY8, GY8.6, G9, or G12 and wondered: What do these codes mean? Which one do I actually need?

This guide will break down the entire family of “G” type sockets, explain their applications, wattage ranges, technologies available (LED, halogen, incandescent, xenon), and even their color options. By the end, you’ll be able to confidently identify the right lamp for your fixture.

What Does the “G” Mean?

The “G” stands for “glass” in the original halogen bulb design, indicating the bulb’s construction type. Over time, it has evolved to simply mean a bi-pin base. The number following the “G” tells you the distance in millimeters between the two pins.

For example:

• G4 = Pins spaced 4 mm apart

• GU5.3 = Pins spaced 5.3 mm apart (the “U” means it has a mechanical lock or “Universal” fitting for better grip)

• GY6.35 = Pins spaced 6.35 mm apart, with slightly thicker pins

This numbering system is the key to matching your bulb to the correct socket.

The Common Bi-Pin Bases Explained

G4 (4 mm)

• Applications: Miniature halogen capsules in cabinet lighting, under-cabinet strips, desk lamps, decorative pendants.

• Tech options:

  • Halogen (classic, crisp white light)

  • LED (energy saving, cool/warm tones)

  • Xenon (longer life, slightly softer than halogen)

• Wattage range: 5–20W halogen, 1–4W LED equivalent.

• Colors: Warm white (2700K), neutral (3000–4000K), daylight (5000–6000K).

GU4 (4 mm, “U” base)

• Similar to G4 but with a GU locking mechanism for secure fit.

• Applications: MR11 spotlights, display lighting, accent lighting.

• Tech options: Halogen, LED.

• Wattage range: 10–35W halogen, 2–5W LED equivalent.

• Colors: Full spectrum depending on LED model.

GU5.3 (5.3 mm, aka MR16 base)

• One of the most common spotlight bases.

• Applications: Recessed ceiling lights, track lighting, landscape garden lights.

• Tech options:

  • Halogen (original standard, very bright)

  • LED MR16 replacements (low power draw, various beam spreads)

• Wattage range: 20–50W halogen, 4–8W LED equivalent.

• Colors: Warm to cool whites, RGB LEDs available.

GY6.35 (6.35 mm)

• Slightly thicker pins than G6.35 for higher wattage stability.

• Applications: Stage & studio lighting, medical & dental lamps, projectors.

• Tech options: Halogen, xenon, LED retrofit.

• Wattage range: 20–100W halogen, 5–15W LED equivalent.

• Colors: Usually white tones, some specialty colored versions.

GU8 (8 mm)

• Less common, but used in niche decorative and technical fixtures.

• Applications: Spotlights, small projectors, display cases.

• Tech options: Primarily halogen, some LED.

• Wattage range: 20–50W halogen.

GY8 (8 mm)

• Similar spacing to GU8 but used in specialty halogen lamps.

• Applications: Medical and optical instruments, display fixtures.

• Tech options: Halogen, xenon.

• Wattage range: 35–100W halogen.

GY8.6 (8.6 mm)

• A robust variant for higher current draw.

• Applications: Commercial lighting, studio fixtures, projectors.

• Tech options: Halogen, xenon, some LED retrofits.

• Wattage range: 35–150W halogen.

G9 (9 mm looped pin)

• Recognizable by its looped wire prongs instead of straight pins.

• Applications: Chandeliers, sconces, modern decorative lighting, small lamps.

• Tech options:

  • Halogen capsules (very bright for size)

  • LED G9 (now widely available, dimmable options too)

• Wattage range: 25–60W halogen, 2–8W LED equivalent.

• Colors: Warm white, neutral white, daylight, RGB LED styles.

G12 (12 mm)

• Larger pin spacing for powerful discharge lamps.

• Applications: Metal halide lamps, high bay warehouse lighting, floodlights, commercial fixtures.

• Tech options:

  • Metal halide (common in stadiums, retail, industrial)

  • Ceramic metal halide (CMH for better color rendering)

  • LED retrofit lamps

• Wattage range: 35–150W metal halide, 15–40W LED equivalent.

• Colors: Warm to cool white, high CRI versions for retail and display.

Choosing the Right Bi-Pin Bulb

1. Measure the distance between pins (this is the code number).

2. Check your fixture type (MR11, MR16, capsule, chandelier).

3. Match voltage – many of these bulbs come in 12V low voltage or 120/220V mains voltage versions.

4. Pick your technology – LED is now the most cost-efficient and long-lasting option.

5. Select the right color temperature – warm for cozy, cool white for work areas, daylight for task lighting.

The world of G-base bi-pin bulbs can look intimidating at first, but once you understand the numbering system, it becomes logical. Whether you’re upgrading halogen spotlights to LED, replacing a decorative chandelier capsule, or maintaining industrial fixtures, there’s a G-base bulb designed for the job.

By remembering that the number simply reflects millimeters between pins, you can solve the mystery of these fittings and ensure your lighting is efficient, safe, and exactly suited to your needs.

If you need any help with anything, please contact us or comment below