Sophisticated order to connect the poles for a jump start of the car battery
Positives first (any order.)
Negative pole to battery negative pole (only on one battery.)
Negative pole to chassis of the car for the remaining battery.
Disconnect the negative pole from the chassis first when you are finished.


You connect the positives first because if you connect the negatives first (the chassis on most vehicles,) then you run a risk of shorting out the batteries if you bump the chassis with the positive cable.  If the chassis aren't connected to each other, then it is much safer to connect the positive cables.
You connect the final negative to the chassis because it will spark when you connect it - that closes the circuit.  A spark can set off hydrogen gas that collects around batteries when they charge or discharge.
You disconnect the negative cable from the chassis for the same reason you connected to the chassis last - it will spark, and you don't want to set off any hydrogen gas that may have accumulated around the batteries during the jump start.

The electrons don't care in the slightest in which order you make the connections, or if one connection is to the chassis.
It is &quot;only&quot; a matter of safety.  Do it right and everyone is happy.  Do it wrong, and you might slag or blow up a battery.

The above applies to the usual negative ground to chassis electrical systems used in modern cars.  Older vehicles might use a positive ground, in which case you need to follow a different sequence.
Check the owner's manual before you try to jump start a car - some may say to not use a jump start at all.