The MathML <mrow> element is used to group sub-expressions, which usually contain one or more operators with their respective operands (such as <mi> and <mn>). This element renders as a horizontal row containing its arguments.
When writing a MathML expression, you should group elements within an <mrow> in the same way as they are grouped in the mathematical interpretation of the expression. Proper grouping helps the rendering of the expression in several ways:
It can improve the display by possibly affecting spacing.
It allows for more intelligent line-breaking and indentation.
It simplifies the interpretation of the expression by automated systems such as computer algebra systems and audio renderers.