(topic equation-d) The Inline Equation element (<equation-inline>) represents an equation that is intended to be rendered inline with its surrounding content. The equation content may be represented in any number of ways, including embedded MathML using the <mathml> specialization of <foreign>, a reference to an image, inline TeX markup, or any other way that an equation might be defined. The equation may include alternative forms, such as both a MathML version and an image. The Block Equation element (<equation-block>) represents an equation to be rendered as a block. Block equations may be numbered. The equation content may be represented in any number of ways, including embedded MathML using the <mathml> specialization of <foreign>, a reference to an image, inline TeX markup, or any other way that an equation might be defined. The equation may include alternative forms, such as both a MathML version and an image. The Equation Number element (<equation-number>) indicates that the equation should be numbered. If the <equation-number> element has empty or whitespace-only content, then the number should be generated. If the <equation-number> element has content other than whitespace, the content should be used as the number. The Equation Figure element (<equation-figure>) represents an equation that may have a title or a description and that may be numbered. When equation figures are numbered they are often numbered separately from figures. Note that block and inline equations may also be numbered. The equation figure element is intended for equations that are not part of the rhetorical flow of a document but that are presented either out of line or otherwise need a title or description. Equation figures that are simply a single equation plus, optionally, a title or description, may use the <mathml> element directly. When the display equation content is more complicated, it should use >equation-block> to clearly distinguish the equation content from non-equation content, such as paragraphs that provide commentary on the equations within the display equation. The equation content may be represented in any number of ways, including embedded MathML using the <mathml> specialization of <foreign>, a reference to an image, inline TeX markup, or any other way that an equation might be defined. The equation may include alternative forms, such as both a MathML version and an image.