aS, SB, BbB,
B
}).
T)* that can be derived
from S.
Consider the linear grammar
({S, B}, {a, b}, S, {S
aS, SB, BbB,
B}).
A derivation using this grammar might look like this:
aS
aB
abB
abbB
abb
Each of {S, aS, aB, abB, abbB, abb} is a sentential form.
Because this grammar is linear, each sentential form has at most one variable. Hence there is never any choice about which variable to expand next.