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These are two different beast actually. The market cross was used in the middle ages to mark the spot of the market, and provided divine protection. In time these became more elaborate. From wood to stone to structures supporting the stone cross. In this way the divine protection became a physical protection as well. But the cross remained the centerpiece, and their builders often chose an architectural form that reflected this fact, square, hexagonal, octagonal etc.
The market hall developed from the offices needed for the official that controlled the market in the later middle ages, growing into the towns civic center, with offices above and a covered hall for the public beneath. Often the market cross and the market hall fused, becoming the central spot in a town and leading to some market halls being constructed in the same form as the elaborate market cross.
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