An acetal is a functional group with the following connectivity R₂C(OR')₂, where both R' groups are organic fragments. The central carbon atom has four bonds to it, and is therefore saturated and has tetrahedral geometry. The two R'O groups may be equivalent to each other or not. The two R groups can be equivalent to each other (a "symmetric acetal") or not (a "mixed acetal"), and one or both can even be hydrogen atoms rather than organic fragments. Acetals are formed from and convertible to carbonyl compounds (aldehydes or ketones R₂C=O). The term ketal is sometimes used to identify structures associated with ketones rather than aldehydes and, historically, the term acetal was used specifically for the aldehyde cases.