Standard state: 1 bar (approximately 1 atm) pressure, specified temperature (usually 298 K), 1 M concentration for solutions. Standard enthalpy of formation: elements in their standard states have $delta_{Hf}$ = 0. Standard state of carbon is graphite (not diamond). Standard state of oxygen is O2(g) (not O3). Standard state of sulfur is rhombic (not monoclinic). Standard state of phosphorus is white phosphorus. These conventions matter when calculating $delta_{Hf}$ — the allotrope must be the standard form.