The right knife
Chef knives come in all shapes and sizes
Generally, my chef knives have a flatter edge profile, making them perfect for slicing and push cutting, rather than rocking. Sizes vary, but the standard chef knife will be 8", or approximately 203mm. For beginners, a 6"- 8" knife is a good starting place, as larger knives can be more difficult to handle and sharpen.
A Kitchen Essential
Just as a good painter has an array of brushes to choose from for specific details, a good cook will have a "kit" of knives. Chef knives are multi-purpose tools, which means that they are able to do a large swath of tasks, but they do have limitations. For any kitchen task that requires special care, there's almost always a corresponding tool.