WEMOS first came to prominence with the D1, an arduino look-alike but with an ESP8266 processor. Then they created the “D1 Mini” which is a postage-stamp-sized form factor for stackable modules. This brings the “shield” concept from Arduino to a smaller footprint.
The “Brain” module holds an ESP-8266 module, one LED and one button.
The modules have an on board “chip” antenna. The “pro” version also has an antenna connector to allow a bigger antenna.
The there are a number of peripheral modules, that can be stacked on top of each other. Alternatively modules may be placed side-by-side using a double or triple base. You can even do both.
Available modules include buttons, lights, screens, speakers, sensors and battery chargers. Prototyping modules let you add custom circuits.
You have three main options for your D1 mini boards:
Solder a stacking header. This has a socket on top of the board, and pins below. You can stack several of these together.
Solder sockets to the top of the board. This forms the “bottom” of a stack.
Solder pins to the bottom of the board. This forms the “top” of a stack.
If unsure, use the the stacking headers; you can still place one of these at top or bottom of a stack.
If height is a critical factor, you can also fit right-angle headers to a board, or simply solder wires directly to the pads.