Given the drama of their names, the actual definition of widows and orphans may be a little disappointing:
A widow is the last line of a paragraph, stranded at the top of a column or page. They should always be avoided.
An orphan is the first line of a paragraph that occurs at the bottom of a column or page. They are not desirable, but neither are they a typographic sin, especially when fixing them may cause more problems than it solves.
Ultimately you're better off fixing widows, orphans, and their wicked cousinthe short exit lineby a judicious use of tracking, hyphenation, and perhaps rewriting. If you plump for the first option, the golden rule is that no one should be able to tell. You don't want your type looking like a concertina: getting tighter here and looser there as you squeeze or pad your text with space. Restrict yourself to no more than 15 (minus 15/1000 of an em) and no one will notice.
To use tracking, select the paragraph that needs attention and track, usually tighter, to cause a different wrap. With the Paragraph Composer on, (the default), this may not work as expected, because InDesign will recompose every line in the paragraph. If you can't get the result you're after with tracking try one of these other options:
To remove all custom tracking or kerning from a range of text, select the text and press Cmd+Option+Q (Ctrl+Alt+Q). Because this sets the automatic kerning method back to Metrics, it won't be much use if you are using Optical Kerning.
Figures 5.15A and 5.15B. Show Custom Tracking/Kerning.
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Tip: Turn on Show Custom Tracking/Kerning
This will highlight in green any paragraphs that have been custom tracked or kerned. This is helpful in a couple of ways. First, you may have inherited a document in progress and want to quickly make sure that the text hasn't been overzealously tracked. Second, if the layout or text of a document is revised so that the tracking in certain areas is no longer necessary, you can easily identify these areas to remove the tracking.
It's good to develop an eye for where you might need to track to gain better text color: Flipping through your pages at a small view percentage with the type greeked can often help to identify the problem areas.
Like it Tight?
Use Word and Letter Spacing for a tight fit not tracking. That way, when you turn on Show Custom Tracking all your text won't be highlighted in greenonly the text that has been locally tracked.