Allen keys

[Tom]

Also worthwhile getting ball ended allen keys, they help greatly in tight spaces.

The only metric marker I know is the Angel, majority are imperial like everyone has mentioned

Ball ended is great for awkward areas. But always be careful about the condition of the Allen key and how it is positioned.
You can easily shred the head of a bolt with these

 

[Tom Allen]

Also worthwhile getting ball ended allen keys, they help greatly in tight spaces.

The only metric marker I know is the Angel, majority are imperial like everyone has mentioned

Try to loosen a bolt with a normal key first, then use the ballend key to remove. I've seen a few ballend keys snap in the head of a bolt, or damage the head of the bolt.