The circuit to be analyzed is described to ngspice by a set of element instance lines, which define the circuit topology and element instance values, and a set of control lines, which define the model parameters and the run controls. All lines are assembled in an input file to be read by ngspice. Two lines are essential:

• The first line in the input file must be the title, which is the only comment line that does not need any special character in the first place.
• The last line must be .end.

The order of the remaining lines is arbitrary (except, of course, that continuation lines must immediately follow the line being continued). This feature in the ngspice input language dates back to the punched card times where elements were written on separate cards (and cards frequently fell off). Leading white spaces in a line are ignored, as well as empty lines.

The lines described in sections 2.1 to 2.12 are typically used in the core of the input file, outside of a .control section (see 16.4.3). An exception is the .include includefile line (2.6) that may be placed anywhere in the input file. The contents of includefile will be inserted exactly in place of the .include line.