Thanks for all the info - what is single strand and single knotting or in other words how does this affect the hair system?
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Single strand means that at the hairline hairs are knotted individually rather than 2 or 3 at a time.
This gives a more natural appearance as hair grow individually and it replicates that better.

Single knotting means that those hairs are only knotted once, instead of double knots which means that
1) the knots are less visible as 'dots' on your forehead and
2) the bleaching is more effective.

When you have very dark hair, these things help to make the hairline look as natural as possible.