PAL

Ptolemaeus Arabus et Latinus

_ (the underscore) is the placeholder for exactly one character.
% (the percent sign) is the placeholder for no, one or more than one character.
%% (two percent signs) is the placeholder for no, one or more than one character, but not for blank space (so that a search ends at word boundaries).

At the beginning and at the end, these placeholders are superfluous.

to achieve a symmetrical appearance ‘to achieve a symmetrical appearance’

English

References: Alm. Toomer: 56
Translation of:
Greek: διὰ τὸ σύμμετρον [Alm. Heiberg: I.10, 47:3]