What is the difference between normally open (NO) and normally closed (NC) contacts?

In ladder logic, normally open (NO) contact passes power when the associated bit is TRUE/ON (contact closes). Normally closed (NC) contact passes power when the bit is FALSE/OFF (contact opens when bit is ON). These terms mirror physical relay contacts. NO is commonly used for start buttons, permissives, and enabling conditions. NC is used for stop buttons, safety interlocks, and fault conditions. The logic should ensure fail-safe behavior - typically NC for safety inputs.