By the way, does the first letter of the *number on the plane* say something of the nationality?
Same as the call sign of Dutch ships start with P
Yep
they 2 first letter are the nation sign, All Norwegian civil plane has first LN- and follow by a number or a combination like this first the land code LN-123 reg number and Sweden has SE- and letters
and that are the same in all country
Yup, true, but they don't have a number, they're 3 other letters, like LN-ABC.
It differs per country.
Netherlands has PH-XXX, Belgium has OO-XXX, while the USA has N000XX or N0000.
The Netherlands can also have PH-0000, but that's for gliders.
There's also registrations like A6-XXX (this being from the UAE).
Russia has 2 different forms, one being VP-XXX and the other being RA-00000.
The VP one is the 'international' one used by their airlines (Aeroflot, S7, Transaero) and RA-00000 one is the one back from the Soviet times.
The weirdest registrations ever belong to the airlines of Panama. Copa Airlines has registrations like HP-0000XXX.
http://www.airliners.net/photo/Copa-Airlines/Embraer-ERJ-190-100IGW-190AR/1587130/L/&sid=2ff6b78f6de8a5484efaefdaf7756413
See that photo