Passing is a form of thread used in metal thread embroidery. It consists of a thin strip of metal wound around a thread core, usually of cotton or silk. Some passings have a white or grey core, others are yellow or orange. This is said to be linked with whether silver (white/grey) or gold (yellow/orange) metal thread is being used. Another explanation is one of economics: an undyed core is cheaper than a yellow/orange dyed one.