Multi-level terminal blocks
Quite often, space inside a cabinet or an electrical panel is limited. To save space, individual plug in terminal block can be connected to form groups or clusters and are generally structured in multi-level configurations.
Each configuration has a different number of layers or levels. For example, a Triple level terminal block has three layers.
Terminal blocks mounting
Terminal blocks are mounted in a variety of ways but by far the most common is by attachment to a DIN Rail.
A DIN Rail is a metal rail of a standard size universally used for mounting devices and components inside electrical panels and equipment racks. 
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Summary
– A terminal block is a component with an insulated frame that has the sole purpose of securing two or more wires together.
– Terminal blocks are classified based on characteristics such as structure, device type, and termination options.
– Terminal Block Connection methods include
Screw-in
Barrier / Europe connectors
Spring-Cage / Spring-Loaded connection
Push-in / Push-Fit
Insulation displacement / Fast connection
Pluggable
Tab
– Terminal Block Types include
Ground
Fused
Thermocouple
– The most common phoenix contact terminal block mounting method is by attachment to a DIN Rail.