Pro Trinket combines everything you love about Trinket with the familiarity of the common core Arduino chip, the ATmega328, with 18 GPIO, 2 extra analog inputs, 28K of flash, and 2K of RAM. The Pro Trinket PCB measures only 1.5" x 0.7" x 0.2" (without headers) but packs much of the same capability as an Arduino UNO. So it's great once you've finished up a prototype on an official Arduino UNO and want to make the project smaller. The Pro Trinket 5V uses the Atmega328P chip, which is the same core chip in the Arduino UNO/Duemilanove/Mini/etc. at the same speed and voltage.