Core 2 Quad processors are multi-chip modules consisting of two dies similar to those used in Core 2 Duo, forming a quad-core processor. While this allows twice the performance to a dual-core processors at the same clock frequency in ideal conditions, this is highly workload specific and requires applications to take advantage of the extra cores. Also, high-end Core 2 Duo processors often operate at higher clock frequencies, so the performance for single-thread workloads would be worse on a Core 2 Quad.
Initially, all Core 2 Quad models were versions of Core 2 Duo desktop processors, Kentsfield derived from Conroe and Yorkfield from Wolfdale, but later Penryn-QC was added as a high-end version of the mobile dual-core Penryn.
Source [Wikipedia]
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