This article is about the older Christmas light bulbs and not about the miniature lights of today.

C6 or Conical 6/8 Miniature Base Lights

That being said, you may have noticed that there is a confusing array of terms used to describe the sizes of vintage Christmas light bulbs. So here's a quick guide to some of the terms used
<ul>
 <li>Base Size: the part that screws into the socket comes in three sizes: miniature, candelabra and intermediate. However, most of us commonly refer to these as C6, C7 and C9.</li>
 <li>Envelope or Glass Size: Size 6=6/8" wide, Size 7= 7/8" wide, 9= 9/8" wide</li>
 <li>Shape of Glass: Describes the shape of the glass envelope. C= conical, G= globe or globular, T= tubular</li>
</ul>
So this means that a C6 light would have a cone shape, be 6/8" wide and have a miniature base. A C7 light would have a cone shape, be 7/8 inches wide and have a candelabra base. A C9 light would have a cone shape, be 9/8" wide and have an intermediate size base.

There's nothing more frustrating than ordering a replacement lamp that is the wrong size. Hopefully the sizes and measurements here will help you sort out the seller's descriptions. If it's not clear, ask before bidding or buying.