What is Principle of Design ?

The principles are concepts used to organize or arrange the structural elements of design. In which these principles are applied affects the expressive contents or message of the work. 

The principles are :
1) Balance
2) Proportion
3) Rhythm
4) Emphasis
5) Unity 

Balance-  Balance is the concept of visual equilibrium, and relates to our physical sense of balance. It is a reconciliation of opposing forces in a composition that results in visual stability. Most successful compositions achieve balance in one of two ways: symmetrically or asymmetrically. Balance in a three dimensional object is easy to understand; if the balance isn't achieved, the object tips over. To understand balance in a two dimensional composition, we must use our imaginations to carry this three dimensional analogy forward to the flat surface.

Proportion- 
Proportion refers to the relative size and scale of the various elements in a design. The issue is the relationship between objects, or parts, of a whole. This means that it is necessary to discuss proportion in terms of the context or standard used to determine proportions.

Rhythm- 
Rhythm can be described as timed movement through space; an easy, connected path along which the eye follows a regular arrangement of motifs. The presence of rhythm creates predictability and order in a composition. Visual rhythm may be best understood by relating it to rhythm in sound.Rhythm depends largely upon the elements of pattern and movement to achieve its effects. The parallels between rhythm in sound/ music are very exact to the idea of rhythm in a visual composition. The difference is that the timed "beat" is sensed by the eyes rather than the ears. Visual rhythm can be created in a number of ways :-
a) Linear rhythm 
b) Alternation
c) Repetition 
d) Gradation

Emphasis- 

Emphasis is also referred to as point of focus, or interruption. It marks the locations in a composition which most strongly draw the viewer's attention. Usually there is a primary, or main, point of emphasis, with perhaps secondary emphases in other parts of the composition. The emphasis is usually an interruption in the fundamental pattern or movement of the viewer's eye through the composition, or a break in the rhythm.
The artist or designer uses emphasis to call attention to something, or to vary the composition in order to hold the viewers interest by providing visual "surprises." Emphasis can be achieved in a number of ways :-
a) Contrast ----use of neutral background / color / texture / shape / size / scale 
b) Repetition
c) Placement in a strategic position 

Unity-
Unity is the underlying principle that summarizes all of the principles and elements of design. It refers to the coherence of the whole, the sense that all of the parts are working together to achieve a common result; a harmony of all the parts.