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Bitcoin blocks are meant to be added every 10 minutes, but sometimes, two blocks can come in at similar times. One block may be sent to half of the network first, while the other block reaches the other half first. Nodes in the network keep both blocks since they are both valid. There are now two valid chains. However, as miners keep adding blocks, they will add them on top of one chain or the other. Eventually, one chain will become longer than the other, and all nodes will adopt the longest chain, abandoning the shorter one. This abandoned chain is a chain of orphan blocks. This orphaned chain is usually only one block long, but there is no limit to the number of blocks that can be orphaned through a reorganization or an extended period of uncertainty, where two valid chains are of equal length.
Because this event causes significant financial detriment to the miner whose block was orphaned, it is important for the network to minimize orphaned blocks. This requires block propagation, the sending of blocks between nodes, to be as fast as possible. This is one reason why blocks are limited in size or weight.