abstract
background
barb
barrier
border
boundary
cage
chain
construction
country
danger
dangerous
design
enclosure
engineering
fence
field
forbidden
freedom
gardening
grass
green
grid
industrial
iron
jail
link
mesh
metal
metallic
military
net
outdoor
outside
pattern
precaution
prevention
prison
protection
safety
security
sky
steel
texture
tube
wall
wire