The dollar dollar function is Prototype's CSS Selector Engine. It returns all matching elements, following the same rules as a selector in a CSS stylesheet. For example, if you want to get all <a> elements with the class "pulsate", you would use the following:

$$("a.pulsate")

This returns a collection of elements. If you are using the script.aculo.us extension of the core Prototype library, you can apply the "pulsate" (blink) effect as follows:

$$("a.pulsate").each(Effect.Pulsate);