17 Combinatorial
Bold italic underline 3D superscript subscript 
(or any other combination, such as global flag values, etc)

Combinatorial explosion: 
Testing all combinations grows EXPONENTIALLY  as the number of items increase

To test all combinations of bold, italic, underlined, 3-D, and superscripted requires 2^5 = 32 tests.
To test all combinations  for 6 items: 2** 6 = 64

One principle:
MOST errors show with pairwise combinations
Up to 90% of combination errors are detected by doing all pairwise combinations

Pairwise combinations are a LOT fewer
6*5 = 30
Pairwise combinations instead of 64

For 10 1024 (2 ** 10) versus 90 (10*9)

Permutations: How many different ways can you arrange N items? (order is significant)

5 items: 5*4*3*2*1 =120 5! Factorial
N!  ways you can order N items

Permutations: select R items out of N
N*(N-1)*….(N-R)

Select 4 of 10: 10*9*8*7
N! (N – R)!
Orderings of 10 items / ordering of items not picked

Combinations: Pick items, order is not important
10*9*8*7/ (4*3*2*1) (ordering of those 4 does not matter)
N!/(N – R)!/R!

Nuer of different COMBINATIONS of 4 items
(order not important)

Software can generate lists of combinations
Calculate permutations of N,
Calculate permutations of R items out of N
Calculate combinations of R items out of N

50 items exhaustve: 2^50, =1,125,899,906,842,624 
while there are 50*49 pairs=2,450 [14 is a typo in book]