THE ADJUSTABLE WRENCH
Adjustable, also called Crescent, wrenches are not usually found as part of a pre-made mechanics tool set. However the adjustable wrench is a tool than can make up for some shortcomings in your combination wrench set and therefor is a valuable tool for any DIY mechanic. These wrenches being adjustable, allow you to adapt to whatever fastener you are using them on. No need to worry weather that bolt is metric or SAE. Just adjust and go. This flexibility covers a variety of wrench sizes. This tool also doesn't take up much space in the toolbox either. It’s nice to have and adjustable wrench that can fill in the gaps where your combination wrench sizes stop. If your combination wrench's only go up to 18mm, then a Crescent wrench that can adjust to more than that is very useful.
However, don’t go using this tool when you have the actual right set of tools. If an adjustable wrench is bad quality or the user doesn’t tighten it down right properly, you can round the edges off a bolt head or nut easily. Then you could be in a mess.
Adjustable wrenches are sold by the length of the wrench. For example, you can buy a 4", 6", or 8" wrench. Of course it's not the handle side that matters but with each handle size there is a larger jaw capacity. A 12" (300mm) adjustable wrench will typically have a 1..5" (38mm) jaw.
When purchasing an adjustable wrench, stay clear of the ratcheting type. They just don't work well. What you should look for is a wrench with as little "play" in the screw mechanism as possible. This is hard to do online, so read reviews or stick to known name brands.
A sloppy mechanism means the adjustment could change when a load on the jaw is applied. This could cause you to round over the corners on a bolt or nut.