Where it hangs
The heavy line at the foot of the ruler goes exactly 50 cm above the floor the child stands on. That rule is printed on the ruler as well, so there is something to read the day it is hung again.
Why 50 cm and not the floor? A skirting board stands proud of the wall, so a print hung down to the floor bows over it — and that bow pulls every mark above it a couple of millimetres down. 50 cm clears every skirting board, radiator ledge and dado there is.