The letter indicates the shape of the base, while the number indicates the size. Always check your lighting fixture before purchasing bulbs. ... These are the basics on light bulb bases. Sizes and shapes do matter – so be mindful before purchasing a new bulb.

Why are light bulbs different shapes?

The reason, Porter says, is the standardization common in manufacturing. When the original bulb was produced, companies needed to make reflector systems—the parts which house and direct the light of the bulb whether in a ceiling or a lamppost—that fit the shape of the bulbs being produced at the time.

Is light bulb shape important?

When choosing a light bulb, it's important that you choose the right shape for the location and situation. Each different bulb shape provides a slightly different spread of light, comes in a different size and complements different fittings and positions.