3. Coordinates, viewport vs viewbox

3.1 The Viewport -3.2 Default User Coordinates and Units of length

3.1 The Viewport
The area of the canvas that your document intends to use is called the viewport.
You establish the size of this viewport with the width and height attributes on the <svg> element, for example
<svg width="200" height="100">

3.2 Default User Coordinates The values of these attributes can be simply a number which is presumed to be in pixels;
However the user can add unit identifiers, for example
<svg width="7cm" height="5cm">

Below is the list of available the unit identifiers:

em
The font size of the default font, usually equivalent to the height of a character
ex
The height of the letter x
px
Pixels
pt
Points (1/72 of an inch)
pc
Picas (1/6 of an inch)
cm
Centimeters
mm
Millimeters
in
Inches
3.3.1 Specyfing the user Coordinates in viewBox with the same aspect ratio as in a vieport

Aspect ratio the same for width and height 4, 5 and 4⋅16 = 64, 5⋅16 = 80.

<svg width="4cm" height="5cm" viewBox="0 0 64 80">
         <rect x="0" y="0" width="100%" height="100%"  stroke="#aaa" stroke-width="0.5" fill="none"/>
         <rect x="0" y="0" width="16" height="16"  fill="orange"/>
         <rect x="48" y="64" width="1cm" height="1cm"  fill="purple"/>
    </svg>
3.3.2 Specyfing the user Coordinates in a viewBox with aspect ratio different from the vieport

Default behaviour

3.4. Preserving aspect ratio

Specifying Alignment for preserveAspectRatio using preserveAspectRatio="alignment [meet | slice | none]"

3.4.1 Preserving aspect ratio with meet Specifier

Tall viewports

preserveAspectRatio="xMinYMin meet"

preserveAspectRatio="xMinYMin meet"

preserveAspectRatio="xMinYMin meet"

Wide viewports

preserveAspectRatio="xMinYMin meet"

preserveAspectRatio="xMinYMin meet"

preserveAspectRatio="xMinYMin meet"

3.4.2 Preserving aspect ratio with slice Specifier

Tall viewports

preserveAspectRatio="xMinYMin slice"

preserveAspectRatio="xMidYMid slice"

preserveAspectRatio="xMaxYMax slice"

Wide viewports

preserveAspectRatio="xMinYMin slice"

preserveAspectRatio="xMidYMid slice"

preserveAspectRatio="xMaxYMax slice"

3.4.3 Using the none Specifier

Tall viewports

preserveAspectRatio="none"

Wide viewports

preserveAspectRatio="none"

3.5 Nested systems of coordinates