The first way to define the blocks is by operating system. One block defines the allowed test cases for WXP and WME. Another defines the allowed test cases for OSX10, and a third defines them for LNX9. The supported browsers are included in their respective blocks. (See the request below.)

The second way to define the blocks is by browser. One block defines the allowed test cases for NS7 and MZL1.5. Another defines the allowed test cases for IE5, and a third defines them for IE6. The supported operating systems are included in their respective blocks. The key here is that any test case defined by a block is allowed. When the blocks are taken together, they define all the allowed test cases and none of the disallowed ones.

It is possible to specify blocks that overlap in various ways also. All of these block definitions are equivalent when they exclude the three unsupported combinations. They lead to test cases covering all the allowed combinations. However different block definitions lead to minor variations in the resulting test cases.

The following request illustrates the constraints example with the blocks defined by operating system. (The text following the plus (+) on the first line of the block is an optional comment.)

Constraints Example - Disjoint Operating System Blocks Operating System Display resolution Connection Browser Application # + (WXP WME) with (IE6 NS7 MZL1.5 IE5) WXP WME low medium high dial-up dsl cable lan IE6 NS7 MZL1.5 IE5 App1 App2 App3 + OSX10 with (NS7 MZL1.5 IE5) OSX10 low medium high dial-up dsl cable lan NS7 MZL1.5 IE5 App1 App2 App3 + LNX9 with (NS7 MZL1.5) LNX9 low medium high dial-up dsl cable lan NS7 MZL1.5 App1 App2 App3

The results table is shown below. It has columns for a Test Case ID, for each test factor and for a Combo Countdown. The top row of the table labels these columns. The next row shows the number of values for each factor and the total number of factor value pairs in the Combo Countdown column.

Test Case ID	Operating System	Display resolution	Connection	Browser	Application	Combo Countdown
	4 Values	3 Values	4 Values	4 Values	3 Values	129
1	WME	low	dsl	IE5	App1	119
2	WME	high	lan	IE6	App2	109
3	WME	medium	cable	MZL1.5	App3	99
4	WXP	medium	lan	NS7	App1	89
5	OSX10	high	dial-up	IE5	App3	79
6	WXP	high	dsl	MZL1.5	App2	70
7	LNX9	high	cable	NS7	App2	61
8	WXP	low	dial-up	IE6	App1	53
9	OSX10	low	lan	MZL1.5	App3	45
10	LNX9	medium	dial-up	MZL1.5	App2	38
11	OSX10	low	cable	IE5	App2	32
12	LNX9	medium	dsl	NS7	App3	26
13	WXP	medium	cable	IE6	App1	22
14	OSX10	low	dial-up	NS7	App1	18
15	WME	high	dial-up	MZL1.5	App1	15
16	OSX10	medium	dsl	IE5	App1	12
17	LNX9	low	lan	NS7	App1	9
18	WME	medium	dsl	IE6	App3	7
19	WXP	low	dial-up	IE5	App3	5
20	WME	low	cable	NS7	App2	4
21	WME	medium	lan	IE5	App3	3