Tax lingo isn’t always intuitive. To help explain your tax return, WSJ breaks down some key terminology—from gross income to above the line deductions—and explains how each connects to one important number: your adjusted gross income. Illustration: MacKenzie CoffmanWASHINGTON—The Internal Revenue Service will begin a pilot program next year to help some taxpayers fill out and file their income tax returns for free online, taking the first step toward building a government-run competitor to TurboTax and H&R Block . The trial run will be welcomed by Democrats and consumer advocates who have been urging the government to give Americans a public tax-prep option. Republicans, meanwhile, have questioned whether the IRS should develop a new tax-filing system.

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