Stippling, traditionally, is very tight small random quilting. In FMQ by machine it usually resembles the "meander" design which is the random wavy line often curving back on itself but not crossing over itself. The Meander is more open and loose. Here is an example of the meander design done as a stipple. It is the quilting done in the bright lime green portion of this example. Usually when a meander becomes a stipple the quilting is no more than 1/4" apart in any direction but is often less than that:
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