How exactly does unit support work?

[kolpo]

Is the number of units I can support for free calculated on a city basis or on a global basis?

For example I'm a communist with 2 towns(each can support 6 free units) in town A do I build 7 units and in town B do I build 0. Shall I then have to pay for the 7th unit because it is above the number of free units of town A or shall the 7th unit be still free because I can support 12 free units in total?

[DrSpike]

Unlike previous incarnations it's global, so it doesn't matter where you produce units.

[vmxa1]

F3 tells you how many units you can support for free (allowed).