Final answer:
Double spacing is the spacing that leaves a blank line between each keyed line, often used in business letters and formal documents for better readability.
Step-by-step explanation:
The type of spacing that leaves a blank line between each line keyed is known as double spacing. This is commonly used in documents like business letters, where the format requires that everything be left-justified and single spaced within the paragraph, with double spacing between paragraphs for clarity and readability.