There are two types of brush heads: clip-on and screw-on or twist-on. You can read more about the differences between these two types of brush heads below.
The attachable brush heads are compatible with most of our Sonicare rechargeable toothbrush handpieces. They fit all Philips Sonicare toothbrush handpieces except the PowerUp Battery and Essence models, which have their own unique brush heads.
The screw-on or twist-on brush heads are our original standard brush heads and are compatible with the old toothbrush models.
You can select which brush head is best for your toothbrush on our Sonicare Brush Head Advisor page

All Philips brush heads fit on it and work well.

Hello Nadine, on the official Philips Sonicare website they talk about the C3 brush head (to be more precise "Sonicare C3 Premium Plaque Defence"). I therefore do not think that the C1 brush head can be used.