I still remember what came up in my mind when I first shopped for a bulb. That was getting a bulb with somewhat right shape like the broken bulb in my apartment.
So nothing technical here and let's start from the shape of a bulb first. I will break the shape into two parts:
Upper portion is defined as "series" (detailed in chart 1) - Nothing critical just space-wise in general
- Lower potion is defined as "base" (detailed in chart 2) - Critical. Simple to say, a light fixture will not shine with a wrong "base"
In everyday uses, for instance in our lighting collections, E26 is the most common bulb base requirement. It just can't go wrong with E26 most of the time.
Here is one exception, our PIPER PENDANT LAMP requires G9 bulb base due to the limited space for this type of fixture. My recommendation is to check the product details for the bulb base before making a bulb purchase.
I hope this starter introduction will help with your bulb purchase.