essential

The plans, which are exceedingly intricate, comprising some thirty separate patents, each essential to the working of the whole, are kept in an elaborate safe in a confidential office adjoining the arsenal, with burglar-proof doors and windows.

The three most essential are gone—stolen, vanished.

They began by using him as a short-cut, a convenience; now he has made himself an essential.

for I dare not hope,” he continued, in a lower and more serious tone, “that he is improved in essentials.”