Parts of a Telegraph (Straight) Key

This is the Bunnell "Triumph" key. First introduced in 1906, this one can be no earlier than 1918, when the wedge lip was added. Click on the pictures to see a larger image.

This is a telegraph key. It is not a key pad, a spark key, a keyer, a sender, a tapper, a transmitter or a sounder. Those are good words and they've all been used on eBay to describe what's pictured here. Regardless, it's still a telegraph key or a straight key or, simply, a key. It is used for sending Morse (named for Samuel F.B. Morse, not Morris) code. Other straight keys may differ in shape or size but, for the most part, they have these components:

Anvil	Same as lower contact
Base	A block of some material (wood, metal, plastic) to which the frame is attached to lend stability and weight to the key.
Circuit Closer	Sometimes called a shorting switch. When engaged, it shorts the wiring posts to each other. For landline telegraphy, this was used to allow electricity to flow to the sounder so messages (telegrams) could be received. In radio work, closing this switch sends a constant tone over the air in a process called tuning-up.
Contact Gap Adjustment	This screw is used to adjust the distance between the lower and upper contacts. The gap affects the distance the knob travels. This in turn has an impact on sending speed and the operator's arm fatigue.
Contact Strap	Provides a connection between the lower contact and the left, rear wiring post. The contact, the strap, and the post are electrically isolated from the rest of the key.
Frame	The heavy stamped or cast metal piece that holds all the other parts of the key.
Front	The end of the key closest to the operator, where the knob is.
Hammer	Same as upper contact.
Knob	A round, usually plastic or Bakelite button used by the operator to move the lever and send Morse code.
Lever	The long, cross-shaped steel bar held on its shorter axis by the trunnions. J.H. Bunnell patented this style of lever in 1881.
Lower Contact	The lower half of the switch that must be opened and closed to send Morse. When the upper contact touches the lower contact, the circuit is closed and a dit or dah is sent depending on how long the circuit is held closed by the operator.
Rear	The end of the key farthest from the operator, the opposite end from the knob.
Spring Tension Adjustment	Regulates how much the lever resists being rotated on the trunnions to make the contacts meet each other, closing the circuit. Also influences how quickly the contacts separate when the operator stops pressing down on the knob.
Trunnions	In the singular, trunnion refers to the transverse, shorter part of the lever. The trunnion screws hold the lever in place and, when properly adjusted, keep the lever from moving side-to-side.
Upper Contact	The upper half of the switch that must be opened and closed to send Morse. When the upper contact touches the lower contact, the circuit is closed and a dit or dah is sent depending on how long the circuit is held closed by the operator.
Vibroplex Wedge Lip	In the early 1900s, with the advent of Horace Martin’s Vibroplex semi-automatic key (bug), operators were able to send much faster code with less wear-and-tear on their arms. The telegraph companies had no interest in buying these so each telegrapher owned his own bug. To connect it to the telegraph system, a metallic wedge was inserted into the key, electrically coupling the bug to the key. Eventually, this lip became standard on telegraph keys.
Wiring Post	For landlines, the key was placed in series with the telegraph line at the wiring posts, forming an open switch on that line. For radio work, the wiring post wires terminate in a standard 2-conductor audio plug which is plugged into the designated jack on the radio transmitter.

THANKS to KD2UJ Telegraphy - Parts of a Key