If the arrow looked like this: → then we would say "If P then Q".
If the arrow was pointing the opposite way like this: ← then we would say "If Q then P"
If the arrow was pointing both ways like this: ↔ then we would say "If P then Q and if Q then P"
P ← Q : If n is a multiple of 4 then n is an even number.
P ↔ Q : If n is an even number then n is a multiple of 4 and if n is a multiple of 4 then n is an even number.
P ← Q : ...it's got to be this!
P ← Q : If AB2 + BC2 = AC2 then B is a right-angle.
P ↔ Q : If B is a right-angle then AB2 + BC2 = AC2 and if AB2 + BC2 = AC2 then B is a right-angle.