An equilateral triangle is a triangle that has three equal sides, an isosceles triangle is a triangle which has only two of its sides equal, and a scalene triangle is a triangle which has all three sides unequal. A right triangle is a triangle which has a right angle, an obtuse triangle is a triangle which an obtuse angle, and an acute triangle is a triangle that has all of its angles acute. Let quadrilaterals other than these be called trapezia . Parallel straight lines are straight lines lying in a plane which do not meet if continued indefinitely in both directions. A circle may be drawn with any center and any distance.
All right angles are equal.
Common Notions Things which are equal to the same thing are also equal to one another. If equals are added to equals, then the results are equal. If equals are subtracted from equals, the remainders are equal. Things which coincide with one another are equal to one another. The whole is greater any of its parts.