In the UK, it is normal to randomly assign fixed couples, and then keep those couples fixed throughout the competition ( for example, if there are multiple rounds ).
2.
These can be performed by fixed couples or in the more sociable " progressive " manner, with the lady moving to the next gentleman in the ring at or near the end of each repetition of the steps.
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But then the writer felt if the show had an odd number ( two guys and three girls or four guys and two girls ), it would add more dynamic and improbability to the equation : there won't be any fixed couples.