How to read railroad diagrams
Each railroad diagram begins with a double right arrow and ends with a right and left arrow pair. Lines beginning with a single right arrow are continuation lines. You read a railroad diagram from left to right and from top to bottom, following the direction of the arrows.
Other conventions used in railroad diagrams are:
Convention | Meaning |
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You must specify values A , B , and C . Required values are shown on the main line of a railroad diagram. |
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You may specify value A . Optional values are shown below the main line of a railroad diagram. |
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Values A , B , and C are alternatives, one of which you must specify. |
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Values A , B , and C are alternatives, one of which you might specify. |
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You might specify one or more of the values A , B , and C . Any required separator for multiple or repeated values (in this example, the comma (,)) is shown on the arrow. |
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You might specify value A multiple times. The separator in this example is optional. |
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Values A , B , and C are alternatives, one of which you might specify. If you specify none of the values shown, the default A (the value shown above the main line) is used. |
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The railroad fragment Name is shown separately from the main railroad diagram. |
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Punctuation and uppercase values | Specify exactly as shown. |