Opposing traveling waves of the same frequency create standing waves with nodes.

Evidence

It can be shown that two sinusoidal wave components having the same mode frequency but travelling in opposite directions on the string result in a standing wave where no net energy transfer takes place. Each subwave reflects back from the terminations and the vibratory energy remains ‘in place’. As Figure 2.7 shows, there are points of maximum vibration as well as points of zero (or minimal) vibration called nodes. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 185

Categorization

• DEFINE: Defines standing waves and nodes, and clarifies the mechanism by which they form.