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size

3 Large columns (col-lg-4)
3 Large columns (col-lg-4)
3 Large columns (col-lg-4)
2 medium columns (col-md-6)
2 medium columns (col-md-6)
4 small columns (col-sm-3)
4 small columns (col-sm-3)
4 small columns (col-sm-3)
4 small columns (col-sm-3)
3 extra small columns that never break (col)
3 extra small columns that never break (col)
3 extra small columns that never break (col)
simple col-md with no numbers
simple col-md with no numbers
simple col-md with no numbers
simple col-sm with no numbers
simple col-sm with no numbers
simple col-sm with no numbers
simple col-sm with no numbers

Note that all columns in a given row must add up to 12. Col-sm-4, for example, represents 4 extra small columns, while col-md-6 represents 6 medium columns.