A Yankee consists of 11 possible bet combinations involving selections in 4 different events.
The bet includes 6 doubles, 4 trebles, and an accumulator
A double is a combination of 2 events, treble is a combination of 3 events and an accumulator in this instance means a combination of all 4 events.
E.g You place a yankee bet on games A,B, C and D.
Let's say your stake is N50, please note that your total stake will be 50 X 11 = 550
This is so because your stake is charged separately for each possible combination and winnings are credited for each correct combination as well.
Your combinations would be arranged thus:
6 doubles
A*B X 50 = Winnings 
A*C X 50 = Winnings
A*D X 50 = Winnings
B*C X 50 = Winnings
B*D X 50 = Winnings
C*D X 50 = Winnings
4 Trebles
A*B*C X 50 = Winnings
A*B*D X 50 = Winnings
A*C*D X 50 = Winnings
B*C*D X 50 = Winnings
A*B*C*D X 50 = Winnings
A minimum of 2 of your selections must be successful to get a return.
An example of a yankee bet is displayed in the attached picture