In chemistry, this is called geminal (from Latin gemini "twins" ) when two identical functional groups (eg, halogens such as fluorine, chlorine or bromine) are bound to the same carbon atom within a chain or a ring system. Differing definitions do not exclude different substituents, which also needs to be noted here that it is identical substituents in the rule.
Further, the relative arrangement of two functional groups, descriptive terms are vicinyl, isolated, and α - and β -position.
1H -NMR spectroscopy
In the 1H -NMR spectroscopy is the coupling of two hydrogen atoms, which are located on the same carbon atom, called geminal coupling. You only occurs when two hydrogen atoms differ in a methyl group stereochemically each other. The geminal coupling constants is referred to as 2Y, since the hydrogen atoms on two bonds link each time. As a function of the other substituents, the geminal coupling constant value between -23 and 42 Hz occupies.