Every episode of Fringe displayed a series of symbols, called glyphs, before each ad break. There were 8 symbols: an apple, a butterfly, a flower, a frog, a hand, a leaf, a seahorse, and smoke. Each glyph was also paired with a little yellow glowing dot in one of its four corners.
Every combination of glyph + dot location corresponded to a letter, meaning the glyphs were a cypher. The glyphs presented over the course of an episode spelled out a word that was related to that episode, the current arc, the current season, or sometimes the show as a whole.
Check out individual episode pages to see what words were spelled out for each episode, starting with season 1 episode 1, 'Pilot'!
The glyphs were also snuck into episodes as easter eggs for eagle-eyed viewers. Scroll down to see the glyph alphabet and every glyph easter egg in the series.