Sideburns are a critical element of a guy's haircut. Shorter sideburns than a style usually takes can lengthen a face and longer sideburns than a style usually takes can shorten a face.
Short styles like a crew cut or ivy league generally take short sideburns.
Short sideburns extend to where the ear cartilage attaches to the skull.
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Medium styles like a regular business men's taper cut take medium sideburns.
Medium sideburns extend to the top of the ear orifice.
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Longer styles that begin to cover the top part of the ear take long sideburns.
Long sideburns extend to the middle of the ear opening.
Styles that cover most or all of the ear take extra long sideburns.
Extra long sideburns extend to the bottom of the ear orifice/top of the ear lobe.
Good Luck!