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Definition

Bit flags are constants which each denotes a single unique case in one situation and can be combined to form different, unique cases. They are called bit flags, because when bits are used to denote a cases, we observe they are indeed single and unique and can be combined to form different unique combinations.

Bit flags are often used as integers:

Bit pattern	- Integer value
0001	- 1
0010	- 2
0100	- 4
1000	- 8

These can be combined to form, for example:

Bit pattern	- Integer value
1001	- 9
0110	- 6
1110	- 14
0101	- 5

Example

When we look at blit flags for example, we see the values:

Constant	- Value	- Description
B_HMIRROR	- 1	- Blit the graph horizontally mirrored.
B_VMIRROR	- 2	- Blit the graph vertically mirrored.
B_TRANSLUCENT	- 4	- Blit the graph with half transparency.
B_ALPHA	- 8	- Blit the graph in some way. (What does this do exactly?)
B_ABLEND	- 16	- Blit the graph using additive blending (nice effect for fire).
B_SBLEND	- 32	- Blit the graph using subtractive blending (nice effect for ghosting).
B_NOCOLORKEY	- 128	- Blit the transparent parts of the graph as black.

These are all single unique cases and can be combined to form different unique cases. For example, when we want a translucent, horizontally mirrored blit operation with use of additive blending, we would do:

B_HMIRROR | B_TRANSLUCENT | B_ABLEND
1         | 4             | 16       = 21

Because the bits are unique, the addition operator can also be used. But when we consider there would be a constant called B_HVMIRROR, which has the value B_HMIRROR|B_VMIRROR (3), the addition operator can't be used all the time:

B_HMIRROR | B_HVMIRROR == 1 | 3 == 3
B_HMIRROR + B_HVMIRROR == 1 + 3 == 4

In conclusion, use the bor operator when dealing with bit flags to be on the safe side.