IUPAC Naming of Spirocyclic compounds:

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Monospiro compounds consisting of only two alicyclic rings as components are named by placing "spiro" before the name of the normal acyclic hydrocarbon of the same total number of carbon atoms.The 4 degree carbon connecting the two rings is called spiro atom.
The number of carbon atoms linked to the spiro atom in each ring is indicated in ascending order in brackets placed between the spiro prefix and the hydrocarbon name. Example:spiro[3.3]heptane

Example 1:SpiroPentane

Example 1: The carbon frame of SpiroPentane contains a total of 5 atoms, hence the root name heptane. This molecule has two paths of 2 carbon atoms each fused with the spiro carbon atom. so the brackets are filled in ascending order: [2.2].
Addition of the prefix spiro gives the total name spiro[2.2]pentane.

Example 2: Spirooctane

The carbon atoms in monospiro hydrocarbons are numbered consecutively starting with a ring atom next to the spiro atom, first through the smaller ring (if such be present) and then through the spiro atom and around the second ring. Example:spiro[3.4]octane

Example 3: spiro[3.5]nona-5-ene

When unsaturation is present, the pattern remain same , but in such a direction around the rings that the double and triple bonds receive numbers as low as possible in accordance with Rule . Example: spiro[3.5]nona-5-ene

Example 4:1-Aza-5-oxaspiro[4.5]decane

Even in presence of heteroatom, the pattern of countomg remain same . Example:1-Aza-5-oxaspiro[4.5]decane