What does the term “on center” mean when talking about carpentry?

I am reading some plans for building a shed. I want to try it but I do not have any carpentry skills. I understand the basic concept of what it means but can someone explain it to me? Here is what it says in the plans…they are talking about the floor of the shed.
After cutting all lumber, mark off your two band boards for joist layout at 12″ on center.
Does that simply mean every 12″ ?

“Does that simply mean every 12″ ?”

Not exactly, it means make a mark every 12 ” on the band boards and put the center of the joist on the mark. However you have to measure in from the edge 3/4″ for the first mark so that it will be in the center of the first joist, do not just hook the tape on the edge of the band board and start measuring.

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