Jointers and planers can be fitted with a spiral cutterhead. These have many separate carbide-tipped blades following a spiral form, instead of a blade across the entire length of the cutter block. There are multiple advantages to spiral cutters. Not only do they give somewhat better planing and jointing results, they are also more convenient to use. Because they are many separate blades, in case you damage one of them, you will only need to change one, instead of the entire blade. Each blade is attached with a screw and has four sharp sides. When the blade gets dull, you simply unscrew it and turn it 90 degrees. You can also check my detailed guide about the pros and cons of spiral cutterheads.