The mnemonic "ACEG" spells out the four notes in the spaces (from bottom to top) between the staff lines in a bass clef, while the staff lines themselves spell out "GBDFA" from bottom to top. Most music students remember these as "All Cows Eat Grass" and "Good Boys Do Fine Always."
In music notation, a bass clef on the staff looks something like this:
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/ \ *
-*-----|---
| *
------/----
/
----/ -----
/
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