Q: What's a good number of teeth in one sided thinning shears to avoid thinning the hair too much?
Second most important factor is the width of the teeth involved. The wider the teeth and spacing between them the larger the chunks of hair are that are removed. Most wide tooth thinning shears are best used for thick naturally curly hair to remove bulk.
When selecting general purpose thinning shears, select a pair that have fairly finely spaced teeth that are fairly slim. Remember when using them that you want to make one cut along the hair length between one-quarter of the way from the scalp and one-quarter of the way from the ends. If the hair is extra thick and long, you can make two cuts along the length at one-third of the length and the two-thirds mark.
The way to avoid over-thinning the hair is to make one pass at a time with the shears. Remember, you can always remove more hair, but once it's cut, it's cut.
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