abstract
background
barrier
black
blue
border
braided
cage
chain
chained
close
design
element
enclose
enclosure
fence
freedom
graphic
green
grid
industrial
industry
iron
isolated
jail
keep
link
mesh
metal
nature
net
old
pattern
prison
protection
safety
security
shape
sky
steel
system
texture
tone
tree
up
wall
white
wire