Question 21

In a B+ tree of order $m$, every non-root internal node and every non-root leaf node must remain at least half full ($\lceil m/2 \rceil$ children or $\lceil (m-1)/2 \rceil$ keys) to guarantee logarithmic height. This $50\%$ minimum occupancy requirement is relaxed only for the root node, which is allowed to have as few as $1$ search key ($2$ child pointers) when it is not a leaf.