A subcircuit that consists of ngspice elements can be defined and referenced in a fashion similar to device models. Subcircuits are the way ngspice implements hierarchical modeling, but this is not entirely true because each subcircuit instance is flattened during parsing, and thus ngspice is not a hierarchical simulator.

The subcircuit is defined in the input deck by a grouping of element cards delimited by the .subckt and the .ends cards (or the keywords defined by the substart and subend options (see 17.7)); the program then automatically inserts the defined group of elements wherever the subcircuit is referenced. Instances of subcircuits within a larger circuit are defined through the use of an instance card that begins with the letter `X'. A complete example of all three of these cards follows:

Example:

* The following is the instance card: 
*
xdiv1 10 7 0 vdivide

* The following are the subcircuit definition cards: 
*
.subckt vdivide 1 2 3 
r1 1 2 10K 
r2 2 3 5K 
.ends

The above specifies a subcircuit with ports numbered `1', `2' and `3':

• Resistor `R1' is connected from port `1' to port `2', and has value 10 kOhms.
• Resistor `R2' is connected from port `2' to port `3', and has value 5 kOhms.

The instance card, when placed in an ngspice deck, will cause subcircuit port `1' to be equated to circuit node `10', while port `2' will be equated to node `7' and port `3' will equated to node `0'.

There is no limit on the size or complexity of subcircuits, and subcircuits may contain other subcircuits. An example of subcircuit usage is given in Chapt. 21.6.