A marble diagram exposes graphically a sequence of observable items in a time fashion.

This simple diagram is the standard de facto of exposing observable sequences when composed with observable operators, such as merge, delay, scan, and so on.

The most practical example is available on http://rxmarbles.com .

A marble diagram showing a merge operation

In the preceding screenshot, we can see two time-based sequences of observable numeric values in the first two lines. The balls are the values. The little vertical lines are the OnCompleted messages that define when a sequence ends its life cycle.

The third line shows the merged observable sequence containing all the values from the first two sequences.

The website offers a great selection of examples regarding the most used reactive operators by using marble diagrams. This is useful to understand the operator job before diving into coding.