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Core.css uses a .row to represent the rows in the container. If you append a class with the largest width to it, it represents a fixed-width row, otherwise it is a full-screen row. .col represents the column in the container, which divides the width of the container into 12 columns, which is what we often call a 12-grid system. .xs-12 represents a column width of 100%. Similarly, .xs-3 represents a column width of 25% because it requires 4 .xs-3 to reach 100% of the 12 grid. Others are analogously from .xs-1 to xs-12.