Each period has a start and an end. If we observe a period as a pair and from one pair either the start or the end is missing, how would we call something, that adds the missing part, making the period whole or complete. period completer or period finisher just doesn't sound right to me. Maybe period adder? In German, I would use the word Ergänzer.
Learn English – A word for the thing that adds the last piece to something, making it complete
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Best Answer
Do you mean period as "a span of time" or in the musical sense? Based on what you have said, the best word I can think of is "complement".
It can be:
See the full definition here:
https://www.merriam-webster.com/dictionary/complement