This article clarifies the difference between composite SIMMs and double-sided SIMMs.

Composite SIMMs

A composite SIMM uses lower-density components to construct a single bank of memory. An example would be a 16MB SIMM that uses 4MB components to construct one or two banks of memory. A non-composite 16MB SIMM uses 16MB components to construct one bank of memory. If a SIMM uses lower-density components to make up a bank of memory, it is a composite SIMM -- and is not recommended nor supported by Apple.

It has always been Apple's position that composite SIMMs are, by design, incompatible with all Macintosh systems.

Double-sided SIMMs

The double-sided SIMMs available for use in the newer Macintosh systems are not the same thing. A double-sided SIMM consists of two separate banks of memory using the correct density components for each bank. An example would be a double-sided 8MB SIMM, which uses 4MB components to construct two separate banks of 4MB each. These SIMMs can be used in Quadra and Centris systems, because the design of the logic board allows each SIMM slot to contain two banks of memory, not just one as in previous systems. If a bank of memory is to contain 4MB, the components for that bank should be 4MB. If the bank is to contain 16MB, the components that make up that bank must be 16MB.