basic
binding
cardinal
chief
compelling
compulsory
concept
crucial
de rigueur
decisive
elementary
essential
exigent
expedient
fundamental
have
imperative
indispensable
mandatory
miscellaneous
momentous
must
necessary
need
needed
obligatory
paramount
prerequisite
pressing
prime
principal
rectangular
red
required
requisite
rubber
sign
significant
stamp
symbol
unavoidable
urgent
vital