Combs are made in a metal mold.
Normally the two halves of the comb - along the spine and up and down the teeth - have a raised edge, this is the seam. A seam is sharp-ish and will by this nature cause some hair breakage.
Seamless combs do not have this raised edge. They are the best type of combs to use because they have the seam removed by having it flattened leading to less hair breakage. When you are shopping for a comb you not only want it to be wide-toothed (i.e. large gaps between the teeth), you also want it to be seamless.
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