The default grid system utilizes 12 columns, making for a 1200px wide container. Below 800px viewport size, the columns become stacked vertically.
Two Columns with the same width
Three Columns with the same width
Four Columns with the same width
Three Columns Different Sizes (3 + 4 + 5)
The grid system is an aid, not a guarantee. It permits a number of possible uses and each designer can look for a solution appropriate to his personal style. But one must learn how to use the grid; it is an art that requires practice.