The orthocentre occurs outside a triangle if and only if the triangle is an obtuse triangle.

The biconditional sentence above compactly states the following four conditional sentences:

• if the orthocentre of a triangle occurs outside the triangle, then the triangle is an obtuse triangle.
• if a triangle is an obtuse triangle, then the orthocentre of the triangle occurs outside the triangle.
• if the orthocentre of a triangle DOES NOT occur outside the triangle, then the triangle is NOT an obtuse triangle.
• if a triangle is NOT an obtuse triangle, then the orthocentre of the triangle DOES NOT occur outside the triangle.

If "P" represents one statement and "Q" represents another statement, then "If P then Q" is called a conditional sentence symbolized as "P → Q"

The following four conditional sentences can be considered:

The biconditional sentence P if and only if Q states that the conditional sentence, its converse, inverse and contrapositive are all true.

The biconditional sentence of this example can be broken up into the two parts below.

P: the orthocentre of a triangle occurs outside the triangle

Q: the triangle is an obtuse triangle

P if and only if Q [i.e. P ↔ Q] results in the following:

• (Conditional sentence: "If P then Q" [P → Q]) if the orthocentre of a triangle occurs outside the triangle, then the triangle is an obtuse triangle.
• (Converse: "If Q then P" [P ← Q]) if a triangle is an obtuse triangle, then the orthocentre of the triangle occurs outside the triangle.
• (Inverse: "If ~P then ~Q" [~P → ~Q]) if the orthocentre of a triangle DOES NOT occur outside the triangle, then the triangle is NOT an obtuse triangle.
• (Contrapositive: "If ~Q then ~P" [~P ← ~Q]) if a triangle is NOT an obtuse triangle, then the orthocentre of the triangle DOES NOT occur outside the triangle.