Greek letters are used to indicate the proximity of a carbon atom to the carbonyl group. The same greek letters also identify the h attached to the carbon (example, hydrogens are attached to carbons). Reactions occurring at the position involves an enol or enolate intermediate. Ketone exists in equilibrium with an enol in the presence of catalytic acid or base. The ketone and enol are tautomers (interconverting constitutional isomers different in the position of h and double bond). The position of the equilibrium generally favors the ketone. In some cases, enol is favored because it is stabilized by a conjugated system and an intramolecular h-bonding interaction. They have the same steps, protonation at the carbonyl group and deprotonation of the position, but the order is different: acid-catalyzed tautomerization, base-catalyzed tautomerization. The a position is deprotonated to form a resonance-stabilized anion. The anionic intermediate is protonated to give the enol.