unless perfectly justified and firmly held
...As a rule, lines of type should be carried in a position which keeps the types themselves horizontal, not perpendicular. In this manner they are supported by the lead or rule underneath. If carried with the feet of the types down they are liable to drop out unless perfectly justified and firmly held.
Many careless compositors persist in correcting lines on the galley in a puttering, botchy manner which results in badly justified lines...
from “Making Corrections,” in A(lexander). A. Stewart, Typesetting : A Primer of Information about Working at the case, justifying, spacing, correcting, making-up, and other operations employed in setting type by hand. Typographic Technical Series for Apprentices—Part II, No. 16. United Typothetae of America (1919) : 40 : link
same (U Michigan copy), via hathitrust : link