Leader lines are those rows of dots, often to be found in tables and contents lists, which carry your eye from one column of information to the next, without the danger of your flipping from one line to the next and thus getting things mixed up. Leader is here pronounced to rhyme with “need”, having nothing to do with the metal used in typesetting.

I think leader lines are really rather ugly, and usually represent an admission of design failure. If the gap is too wide for the eye to bridge reliably, then reduce the gap by indenting the lines left and right and/or increase the line spacing so that the eye can more reliably move from line to line without error. There’s almost always a work-around, and surviving leader lines suggest to me a lazy or ignorant designer.

Here’s a ludicrous example from an 1894 edition of Balzac in translation printed in Philadelphia by George Barrie and Son. The line isn’t even long enough to make leader lines helpful: the compositor has just put them there out of habit or because he thinks that’s what you have to do, there can be no other reason. Look at the crazy one for page 272!

One basic principle of good book design and composition is that nothing should be included that does not have a function. These leader lines serve no purpose.

Their use in technical drawing to establish a link between a drawn object and text describing it is a horse of a different color, and as such is of course altogether acceptable.