abstract
background
barb
barbed
barrier
black
border
boundary
bright
color
concentration
copy
country
daylight
death
editorial
europe
extermination
extinction
fence
flare
forbidden
frame
freedom
gas
gold
iron
jail
mesh
metal
military
old
pattern
power
prevent
prison
protection
safety
secure
silhouette
sky
space
steel
sunset
symbol
system
wall
war
wire
yellow