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Two-piece Crank
A style of crankset in which the axle and right crank/chainwheel are a single part, and the left crank attaches to the axle either with a cotter or by tightening a pinch bolt that holds the crank to a splined axle. These were formerly usually found on low-quality bicycles and exercise machines of European manufacture. The most common type is known as "Thompson."
Most crank sets are three-piece, there are also one-piece crank sets.
Shimano revived the two-piece design with the 2004 Dura-Ace "Hollowtech II ® " crank and bottom bracket. These use external-bearing bottom brackets and large-diameter hollow shafts.
This system has "trickled down" the Shimano line in later years, and is gradually replacing three-piece cranks on most better quality bikes. Other manufacturers have also adopted this system, which seems to be becoming the new de facto standard.
