https://en.wikipedia.org/wiki/HHScience and technology
HH Household
Hh, a signalling molecule in Drosophila named for the Hedgehog signaling pathway
hh blood group, a rare blood type
Henderson–Hasselbalch equation, in biology and chemistry
Herbig–Haro object, in astronomy
Hitchhiker Program, a NASA program established in 1984
Hodgkin–Huxley model, an electrical model of neurons
Microsoft Compiled HTML Help (hh.exe)
Transportation
HH (Court Street Shuttle) a defunct line on the New York City Subway from 1936 to 1946
HH (Rockaway Shuttle), from 1962 to 1972
HH Ferry route between Helsingborg, Sweden and Helsingør, Denmark
Air Hamburg, a German airline based in Hamburg (IATA code HH)
Other uses
hh (digraph), in several languages
HH, a brassiere cup size
hh, an abbreviation for "hands high" when referring to the height of a horse
HH, short for Hand-Held, a term in film production among camera staff
Hamburg, Germany (ISO 3166-2 code DE-HH) and HH ("Hansestadt Hamburg"), vehicle licence plate prefix
His Highness, or Her Highness, a formal address
HH, the production code fo