Parallelism refers to the practice of using similar patterns of grammar, and sometimes length, to coordinate words, phrases, and clauses.

Parallel construction is especially important in bulleted lists. The table shows some unparallel structures and how they can be made parallel with a little rewording.

Parallel (do) Unparallel (don’t)
  1. Mount the panel.
  2. Install the battery.
  3. Wire the keypad.
  1. Mount the panel.
  2. Battery installation.
  3. Wiring the keypad.
The system sends a message to the call center, which calls the customer and tells about the burglary. The system sent a message to the call center, which calls the customer and will tell about the burglary.
The system tried to send alarm messages that were precise and accurate. The system tried to send alarm messages that had precision and were accurate.
You need to install the panel, wire the sensors, and test the phone line. You need to install the panel, wire the sensors, and you must test the phone line.