many coastal areas have two high tides and two low tides every lunar day. A lunar day is about 24 hours and 50 minutes. This is show long it takes for a spot on Earth to relate from an exact point under the moon back to that same point. This should cause the time between two hight tides to be 12 hours and 25 minutes. This is half of lunar day.
But not all coasts have two high tides and two low tides every lunar day because there are many other factors that affect tides. As a result,three basic tidal patterns exist on Earth : semidiurnal,mixed semidiurnal,and diurnal.