Which of the following is NOT a major American pruning system for grapevines?

The major American pruning systems for grapevines have specific characteristics and practices that define them. The Double Cordon is indeed one of the recognized systems utilized in grapevine cultivation. It involves training two horizontal arms (cordons) on either side of the vine, allowing for a structured approach to vine growth and fruit production.

In contrast, the term "Double Cordon" as an option is not a primary recognized system and doesn’t align with the common classifications used when discussing American pruning styles. Instead, options like the Umbrella-Kniffin, Kniffin, and Geneva Double Curtain are established pruning techniques. The Umbrella-Kniffin method, for example, utilizes a specific umbrella shape to manage and optimize sun exposure for the grapes, while the Geneva Double Curtain technique is notable for its effectiveness in maximizing yield in certain vineyard setups.

By understanding the definition and characteristics of recognized pruning systems, it's clear that "Double Cordon" does not fit the typical major pruning methods for grapevines in the context of American viticulture.