Because thick line combination horizontal line gives the idea bird combination line gives sense aboutvertical line design combination vertical line the repetitionthis gives the sense wave
This gives the imagination of longitudinal space.
This also gives the effect of way out.
It gives the illusion of STROCK because the width and length of line is decreases together.
It also gives the imagination of roof .
Chair surface very strong at middle part, because of thick line.
this combination of line gives sense about
VERTICAL LINE DESIGN:
(III.) 
(IV.)