Cindy,
Not a "Dumb Question" at all!
Simple answers:
For Grounded Systems; The highest Voltage between an Ungrounded Conductor and the System's Grounded Conductor equals the "Potential To Ground" (Voltage To Ground)
Example Systems:
120/240 VAC 1 Phase 3 Wire - grounded center tap: Voltage to ground = 120 VAC (< 150 VAC),
208Y/120 VAC 3 phase 4 Wire Wye: Voltage to ground = 120 VAC (< 150 VAC),
120/240 VAC 3 phase 4 Wire Delta: Voltage to ground = 120 VAC (on 1 phase 3 wire circuits), 208 VAC for L-L / 3 phase circuits (> 150 VAC),
240 VAC 3 Phase 3 Wire Corner Grounded Delta: Voltage to ground = 240 VAC (> 150 VAC),
480Y/277 VAC 3 Phase 4 Wire Wye: Voltage to ground = 277 VAC (> 150 VAC),
480 VAC 3 Phase 3 Wire Corner Grounded Delta: Voltage to ground = 480 VAC (> 150 VAC),
600 VAC 3 Phase 3 Wire Corner grounded Delta: Voltage to ground = 600 VAC (> 150 VAC).
For Ungrounded Systems:
"At Least" the L-L Voltage would be considered the Voltage to Ground.
Actual Voltage to Ground - as applied across an unintentional connection (AKA Personnel), is what Potential Difference is required to push a current across an Impedance (resistance of the unlucky person + the reactance between the circuit and the transformer) via the Capacitively Coupled Circuit.
As you know, many long winded threads have been posted here discussing this, so no need for me to blow out more baloney!
Scott s.e.t.