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  • Dustin Waybright thinks this is interesting:

The clustered index key columns are used by all nonclustered indexes internally as the means to locate rows in the table. So if you define a clustered index on a column x, and nonclustered indexes—one on column a, one on b, and one on c—your nonclustered indexes are internally created on column (a, x), (b, x), and (c, x), respectively.

From

Cover of Training Kit (Exam 70-461): Querying Microsoft® SQL Server® 2012

Note

Clustered indices are combined with nonclustered indices when nonclustered indices are generated.