Also referred to as allen wrench (or misspelled as alan) and it is a necessity for your toolbox. A good set of metric allen wrenches is likely the most essential tool for modern bicycles. They are hexagonal rods of varying width (1mm – 10mm) designed to fit the many bolts on modern bikes from water bottle cage, to headset, to crank bolts. If you find yourself working on bikes for a significant portion of your week, a nice set of allen keys is a must. I like the L shaped ones with a nice rubber or plastic molded grip at the bend. For many a compact multi-tool that includes the most common allen key sizes will be all that is required. Alternatives to allen wrenches include hexagonal rods of varying width by mm or even a good set of metric hex wrenches will do the trick. Just buy a set.