background
barbed
barbwire
barrier
blue
border
boundary
chain
chainlink
clouds
concept
confinement
convict
cutout
detain
enclose
enclosure
exclusion
fence
incarceration
jail
link
linkage
maximum
mesh
metal
points
pow
prison
prisoner
protection
restriction
security
sharp
silhouette
sky
skyscape
steel
strong
symbol
system