Hope it's helpful! These are just the observations that helped me the most, and the ones that seem to get overlooked.

This stuff should work in any style. :3

More information can be found in the drawings (and in the drawings linked to below) than in the writing alone. For example, look at how the fingernail affects the angle of the tip of the finger when viewed from the side. _{(See, it'd just be confusing to write about that kind of thing. XD )}

EDIT 1:
That's how high MY thumb reaches. (When lying flat.) Many people have longer ones. (I wasn't aware that it was so many before I made this tutorial, or I would have averaged the length.) This is where you need to combine tutorial learning with personal observation, because everyone observes slightly different things no matter what, partly because natural organisms vary widely in shape and proportion.

EDIT 2:
If you're looking for a longer hand tutorial, `alexds1 has a great one with a lot of helpful illustrations and examples: [link]
And for a more advanced hand/arm reference, this tutorial by ~Snigom is fantastic: [link]

And here are some hand drawings for reference and inspiration!:
by me: [link] [link] [link]
by other deviants: [link] [link] [link] , [link] [link] [link] , [link] [link] [link] , [link]
Sarah Simblet: [link]
Egon Schiele: [link] [link] [link] [link]
Araizumi Rui: [link] [link] [link]