ff or ff.

1. Fecerunt (Latin), (they) did it or made it
2. Folios (printing)
3. Following pages, lines, etc
4. Fortissimo (music)

FF

1. Form feed (computing)
2. French franc or francs

• lightning chess
• lightning conductor
• lightning conductors
• lightning rod
• lightning rods
• lightnings
• lightning strike
• lightning strikes
• lightning-tube
• lightning-tubes
• light of nature
• light-o'-love
• light-o'-loves
• light opera
• light-organ
• light-organs
• light out
• light pen
• light pens
• light pollution
• light-proof
• light railway
• light railways
• lights
• lightship