I don't understand this table from Wikipedia completely:
row1 1 1 one-to-one person ←→ weight row2 0..1 1 optional on one side one-to-one date of birth ←→ person row3 0..* 0..* optional on both sides many-to-many person ←→ book row4 1 1..* many-to-one person ←→ birth place
row2: I think every person has exactly one date of birth. I don't understand this
row4: I think every person has exactly one birth place. I don't understand this.
What am I missing?