Why Voltage is a difference in potential between two points, so a voltmeter must touch both ends of the component at once, which means connecting it across (in parallel with) the part. A voltmeter has very high internal resistance, so bridging it across a circuit draws almost no current and barely disturbs what you are measuring. Ammeters are the opposite: low resistance, inserted in series so all the current flows through them.
Watch out The series choice describes how an ammeter is connected; wiring a high-resistance voltmeter in series would block current and give a meaningless reading. Quadrature and in phase refer to signal phase relationships, not meter hookups.
Volts across, amps through. Voltmeter goes in parallel, ammeter breaks the circuit.