Combinations of Choices
If there are n choices for one type and m choices for second type, then number of combinations for both types is the product of choices:n*m.
Example:
If there are 3 kinds of breads to choose from and 4 kinds of meats, then we can make 3*4=12 different sandwiches.
If an alphabet has 26 letters, then the number of possible 2 letter word is 26*26=676.All of them may not be vaild words.