From BR Bullpen
A professional baseball game has nine scheduled innings. The game is sometimes scheduled for 6 or 7 innings in youth leagues or high schools; in the minor leagues, double-headers typically consist of two seven-inning games. An inning consists of 6 outs or 3 in each half inning. The visiting team bats first in each inning (the top half of the inning) and the home team bats second (the bottom half).
Games may last for more or fewer than their scheduled number of innings. Games are commonly shortened by part of an inning by a rule saying that the home team does not need to complete the final half inning of the game if it has the lead. Much less often, games may called by the umpire-in-chief because of unsafe playing conditions caused by bad weather or darkness or in the event of a forfeit. A called game is considered to be official if it has lasted for at least 5 innings, or 4 1/2 innings if the home team has the lead. A game that is tied at the end of the scheduled number of innings will go into extra innings. An extra innings game continues until the visiting team has the lead at the end of a complete inning or the home team takes the lead in the bottom half of the inning.
 Inning Pitched
Innings Pitched (IP) measures the length of a pitcher's appearance by outs. If a pitcher gets 1 out it's 0.1 innings (also written 0.3, both meaning 1/3 of an inning), 2 outs is 0.2 innings (also written 0.7) and 0 outs is 0 innings. Until 1982, innings pitched used to be rounded to the nearest full inning (e.g. 13.2 innings pitched became 14, and 13.1 became 13). This practice caused a controversy in the attribution of the 1981 AL ERA title and was scrapped (see below). The practice had been adopted before the invention of the pocket calculator and the spread of computers and no longer made any sense. Encyclopaedias now list full innings pitched, including fractions, for the period before 1982, but older sources of data will often have rounded innings, and as a result pitchers' ERAs will differ slightly.
 1981 AL ERA Title
The ERA race between the Orioles' Sammy Stewart and the Athletics' Steve McCatty was extremely close. When using full inning totals, Stewart would have won the ERA crown by one percentage point, but with innings rounded as was the practice at the time, McCatty came out ahead, 2.32 to 2.33.