You need a mainline for this pattern to work. It doesn't necessarily have to be the trunk though (although the trunk would be a pretty natural choice in most cases).
这个模式需要一条主线才能进行下去，不过不必是主干(虽然在大多数情况下，使用主干是很自然的选择)。
Written English is more or less the same in both Britain and America, though (although) there are some spelling differences.
虽然美国英语和英国英语存在着一些差别，但在笔头上大体是一样的。
Written English is more or less the same in both Britain and America, though (although) there are some spelling differences.
虽然美国英语和英国英语存在着一些差别，但在笔头上大体是一样的。
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