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

• prisoner of war
• prisoners
• prisoner's base
• prisoners' base
• prisoner's bases
• prisoners of war
• prison house
• prison houses
• prisoning
• prisonment
• prisonments
• prison officer
• prison officers
• prisonous
• prisons
• prison ship
• prison ships
• prison van
• prison vans
• prison visitor
• prison visitors
• prison yard
• prison yards
• prissier
• prissiest