平行線と横断線の計算機

The Parallel Lines and Transversals Calculator is the most complete tool on this site for working with the 8 angles formed when a transversal crosses two parallel lines. You enter just ONE known angle and select its position (1-8) on the standard diagram. The calculator returns all 8 angles with their relationships labeled — corresponding, alternate interior, co-interior, vertical, and linear pair. This tutorial covers the standard angle numbering convention, all the relationships, and how to use the results in proofs.

The 8-angle setup

Two parallel lines (an upper line and a lower line) are crossed by a single transversal. At each intersection, 4 angles form, for a total of 8.

Standard numbering (clockwise from upper-right):

• Upper intersection: ∠1 (upper-right), ∠2 (lower-right), ∠3 (lower-left), ∠4 (upper-left)
• Lower intersection: ∠5 (upper-right), ∠6 (lower-right), ∠7 (lower-left), ∠8 (upper-left)

This numbering convention is used in most textbooks and is what this calculator expects.

All the relationship rules

Equal pairs (when lines are parallel):

• Corresponding: ∠1=∠5, ∠2=∠6, ∠3=∠7, ∠4=∠8 (same position at each intersection)
• Alternate interior: ∠3=∠5, ∠4=∠6 (between parallel lines, opposite sides of transversal)
• Alternate exterior: ∠1=∠7, ∠2=∠8 (outside parallel lines, opposite sides)
• Vertical (at each intersection separately): ∠1=∠3, ∠2=∠4, ∠5=∠7, ∠6=∠8

Supplementary pairs (sum to 180°):

• Co-interior / same-side interior: ∠3+∠6=180°, ∠4+∠5=180°
• Co-exterior: ∠1+∠8=180°, ∠2+∠7=180°
• Linear pair (at each intersection): ∠1+∠2=180°, ∠2+∠3=180°, etc.

The checkerboard pattern

Because of all these relationships, the 8 angles have only TWO distinct values. Once you know one angle θ, all 8 angles are either θ or 180° − θ in a checkerboard pattern around the figure.

For example: if ∠1 = 65°, then:

• ∠1 = ∠3 = ∠5 = ∠7 = 65° (all equivalent via vertical / corresponding / alternate)
• ∠2 = ∠4 = ∠6 = ∠8 = 115° (the supplements)

Worked example

You're told ∠3 = 78° (one of the "interior" angles on the lower-left of the upper intersection). Find all other angles.

Pattern: angles equal to ∠3 (= 78°): ∠1, ∠3, ∠5, ∠7.
Angles supplementary to ∠3 (= 102°): ∠2, ∠4, ∠6, ∠8.

So all 8 angles are determined: 4 of them are 78°, the other 4 are 102°.

The converse theorems

Each "if parallel then angle relation" rule has a converse: "if angle relation then parallel". These are powerful tools for PROVING parallelism:

• Converse of corresponding: if corresponding angles equal → lines parallel.
• Converse of alternate interior: if alternate interior angles equal → lines parallel.
• Converse of co-interior: if co-interior angles supplementary → lines parallel.

In a proof: showing that any one of these conditions holds is sufficient to conclude that two lines are parallel.

Using the calculator in proofs

When constructing a two-column proof that involves parallel lines:

1. Identify the parallel lines and transversal in the figure.
2. Number the 8 angles per the standard convention (or label them with your own labels).
3. Use the calculator to confirm which pairs are equal and which are supplementary.
4. Cite the specific relationship by name in your "Reason" column: "Alternate interior angles, AB ∥ CD" or similar.

The calculator output also identifies which postulate (ASA, AAS, etc.) might apply if the figure includes triangles with parallel-line sides.

Where these relationships come from

The foundational fact is the parallel postulate (Euclid's 5th postulate or its modern equivalents): given a line and a point not on it, exactly one line through the point is parallel to the given line.

From this single postulate, all the parallel-line angle theorems follow as consequences via the linear-pair and vertical-angle theorems applied at each intersection.

The "F", "Z", and "C" patterns

Geometry teachers often introduce the angle relationships visually:

• "F" pattern: the corresponding angle relationship looks like an "F" (or backwards F) when traced with the transversal.
• "Z" pattern: alternate interior angles look like a "Z" (or backwards Z).
• "C" pattern: co-interior angles look like a "C" (the two angles on the same side of the transversal).

These shapes are visual mnemonics — useful for quickly identifying which relationship applies in a figure.

Real-world applications

• Construction: ensuring walls / beams are parallel by verifying the angle relationships from a transversal brace.
• Drafting and CAD: precision angular measurement based on parallel-line geometry.
• Cartography: latitude lines crossing meridians follow these angle relationships approximately (on small scales).
• Engineering trusses: parallel chord trusses use these relationships in their angle analysis.
• Geometry proofs: the most-used "free angle equalities" in standard textbook proofs.

Common mistakes

• Treating co-interior as equal. Co-interior pairs are SUPPLEMENTARY (180°), not equal. This is the single most common student error.
• Confusing position 1 with position 5 (or other corresponding pairs). Corresponding angles look identical but are at different intersections. Their relationship is "equal", not "same angle".
• Forgetting parallel is a requirement. All these relationships ONLY hold when the two lines crossed by the transversal are parallel. Without parallelism, all 8 angles can be anything.
• Using the calculator on non-parallel-line figures. The output assumes parallelism. Apply to a non-parallel-line figure and the results are nonsense.