How do blood clots usually form?

Blood clot causes: We doctors are taught to remember virchow's triad: blood vessel injury, stasis, and hypercoagulability. The first refers to things like surgery and catheters, the second to slow blood flow due to immobility or compression on a blood vessel, and last to things that make the clotting system more active like birth control pills and hereditary disorders that make the blood more likely to clot.