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Closed figure
A figure with no open ends is a closed figure.

Regular closed figures
A closed figure in which all the sides and angles are equal.

Simple curve
A curve does not cross it self is called simple curve.

Simple closed curve
A curve with no open ends and does not cross it self is called simple closed curve.

Interior region
The portion of the plane inside the curve is called interior region.

Exterior region
The portion of the plane outside the curve is called exterior region.

Perimeter is the distance covered along the boundary forming a closed figure when we go round the figure once. The perimeter of a regular closed figure is equal to the sum of its sides. The concept of perimeter is widely used in real life.
1) For fencing land. 
2) For building a compound wall around a house.

Perimeter of a rectangle
= Length (l) + Breadth (b) + Length (l) + Breadth (b)
= 2 ( l + b)

Perimeter of a square
= S + S + S + S
= 4 × S.

Equilateral triangle
A triangle with all its sides and angles equal is called an equilateral triangle.
The perimeter of an  equilateral triangle with the side 'a' = a + a + a = 3 × a

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