[b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/defI10.html]Definition 10[/url].[br][/b]When a straight line standing on a straight line makes the adjacent angles equal to one another, each of the equal angles is [i]right,[/i] and the straight line standing on the other is called a [i]perpendicular[/i] to that on which it stands.[br][br][b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/post4.html]Postulate 4[/url].[br][/b]That all right angles equal one another.[br][br][color=bb0033][b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/propI12.html]Proposition 12.[/url][/b][/color][color=bb0033]To draw a straight line perpendicular to a given infinite straight line from a given point not on it.[/color]

[b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/defI10.html]Definition 10[/url].[br][/b]When a straight line standing on a straight line makes the adjacent angles equal to one another, each of the equal angles is [i]right,[/i] and the straight line standing on the other is called a [i]perpendicular[/i] to that on which it stands.[br][br][b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/post4.html]Postulate 4[/url].[br][/b]That all right angles equal one another.[br][br][color=bb0033][b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/propI11.html]Proposition 11.[br][/url][/b][/color][color=bb0033]To draw a straight line at right angles to a given straight line from a given point on it.[/color]

[b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/propI13.html]Proposition 13.[/url][/b]If a straight line stands on a straight line, then it makes either two right angles or angles whose sum equals two right angles.[br][br][b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/propI14.html]Proposition 14.[/url][/b]If with any straight line, and at a point on it, two straight lines not lying on the same side make the sum of the adjacent angles equal to two right angles, then the two straight lines are in a straight line with one another.[br][br][b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/propI15.html]Proposition 15.[/url][/b]If two straight lines cut one another, then they make the vertical angles equal to one another.[br][b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/propI15.html#cor]Corollary.[/url][/b] If two straight lines cut one another, then they will make the angles at the point of section equal to four right angles.[br]

[b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/defI22.html]Definition 22[/url].[br][/b]Of quadrilateral figures, a [i]square[/i] is that which is both equilateral and right-angled; an [i]oblong[/i] that which is right-angled but not equilateral; a [i]rhombus[/i] that which is equilateral but not right-angled; and a [i]rhomboid[/i] that which has its opposite sides and angles equal to one another but is neither equilateral nor right-angled. And let quadrilaterals other than these be called [i]trapezia.[/i]

[b][url=https://mathcs.clarku.edu/~djoyce/java/elements/bookI/defI23.html]Definition 23[/url].[br][/b][i]Parallel[/i] straight lines are straight lines which, being in the same plane and being produced indefinitely in both directions, do not meet one another in either direction.