One of the keys to selecting rows and columns simultaneously is to understand the use of the comma in the brackets. The selection to the left of the comma always selects rows based on the row index. The selection to the right of the comma always selects columns based on the column index.

It is not necessary to make a selection for both rows and columns simultaneously. Step 2 shows how to select all the rows and a subset of columns. The colon represents a slice object that simply returns all the values for that dimension.