How many angles are formed when two lines in the third intersect?

When two straight lines of the third intersect, eight angles are formed, we denote them as ∠1, ∠2, ∠3, ∠4, ∠5, ∠6, ∠7 and ∠8.
Of these eight corners, pairs can be distinguished:
1) criss-crossing interior angles: ∠3 and ∠5, ∠4 and ∠6;
2) criss-crossing external corners: ∠1 and ∠7, ∠2 and ∠8;
3) one-sided internal corners: ∠4 and ∠5, ∠3 and ∠6;
4) one-sided outer corners: ∠1 and ∠8, ∠2 and ∠7;
5) the corresponding angles: ∠1 and ∠5, ∠2 and ∠6, ∠4 and ∠8, ∠7 and ∠3.
If two straight lines are parallel, then the cross-lying and corresponding angles are equal, and the sum of one-sided angles is 180 °.