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About these units
Drop (drop)
The drop is one of the oldest fluid measures and originally referred simply to the amount of liquid that naturally forms at the end of a dripping vessel. Because drop size depends on viscosity, surface tension, temperature, and orifice size, early medicine found drops inconsistent and unreliable. Modern medicine and chemistry sometimes define a drop as 0.05 mL, but this is only a convention used for standardized droppers—real drops can vary significantly. Despite its imprecision, the drop survives in everyday language, aromatherapy, essential oils, and household instructions ("add a few drops"). It exemplifies humanity's earliest attempts to quantify small volumes before scientific instrumentation enabled precise micro-measurement.
Cord (cd)
The cord is a large unit used in North America to measure firewood, defined as a neatly stacked pile measuring 128 cubic feet (usually arranged as 4 ft × 4 ft × 8 ft). Its strict definition helps prevent fraud in firewood sales and ensures fairness in trade. The cord accounts for both wood and air space, though the amount of actual wood can vary depending on how tightly it is stacked. Despite the prevalence of metric units worldwide, the cord remains a robust standard in forestry, firewood commerce, and rural heating practices across the US and Canada.