Spell Mechanics

[Herald Talia]

No: HPFGUIDX 24489

What are some of the mechanics of spell casting in the Potterverse? 
The Summoning Charm, for example - is it limited to line-of-sight? Mrs.
Weasley certainally used it to find Ton-tongue Toffees that she didn't
see. If it's more long distance, why couldn't Harry just Summon the diary
when it was stolen from him? (CoS - when Ginny steals TR's diary back
from Harry.)
Robyn

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Donald heard a mermaid sing, Suzy spied an elf.
But all the magic I have known,
I've had to make myself- Shel Silverstein
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