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Click on any section that is puzzling you. The entries on the 1040 are usually transferred from 'Schedules' that are attached to your 1040.
Why are these forms called Schedules? The word derives from the Middle English cedule or sedule, and originally referred to a written document, usually a scroll or slip of parchment. There's also a more specific meaning for schedule that goes back hundreds of years: A supplementary or explanatory form attached to a document such as a will or deed.
The “schedular system” of taxation, with different rates applied to different types of income, can be distinguished from a “global system”. A non-scheduler system might, for example, tax investments, social security and wages at identical rates. Most countries have a combination of the two systems.