C9 Christmas light bulbs are the larger of the two most common Christmas Light Bulbs. They feature a intermediate base, and look great indoors and out. All American Christmas Co. offers a variety of C9 Christmas lights, including Steady Burn, Twinkle, and Opaque bulbs. Like the name suggests, Steady Burn bulbs run constantly, while Twinkle bulbs randomly twinkle (or flash). Opaque bulbs are the classic painted Christmas lights that offer a soft festive glow.