# Definition: 1.19: Rectilinear Figure, Sides, n-Sided Figure

Rectilinear figures are those (figures) contained by straight lines: trilateral figures being those contained by three straight lines, quadrilateral by four, and multilateral by more than four.

### Modern Definition

A n-sided figure (or a rectilinear figure) is a figure given by a finite number $$n\ge 3$$ of segments $$\overline{A_1B_1},\overline{A_2B_2},\ldots,\overline{A_nB_n}$$, also called its sides, in such a way that

1. $$A_1=B_2, A_2=B_3, \ldots, A_{n-1}=B_n,A_{n}=B_1$$, and that
2. all sides do not have any other points in common except their endpoints, and
3. each endpoint is the endpoint of exactly two sides.

We denote n-sided figures beginning with an arbitrary point and listing all points counter-clockwise.

### Example

An 11-sided figure $$B A K J I H G F E D C$$: |

Chapters: 1
Corollaries: 2 3 4 5
Definitions: 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Proofs: 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55
Propositions: 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70

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### References

#### Adapted from CC BY-SA 3.0 Sources:

1. Callahan, Daniel: "Euclid’s 'Elements' Redux" 2014