|Specifics: Because large wads of text without images scare people|
The most fundamental and defining gimmick of most Transformer toys is, surprisingly, transformation. Nearly every toy marketed as a Transformer can change from one form to another thanks to the brilliance of Hasbro and TakaraTomy's engineers and designers.
Types of Transformation
Because many beasts have four limbs, and most Transformer robots have four limbs, beast-to-robot transformations are sometimes not more complicated than "stand up and switch heads". For most incarnations of Cheetor, for example, the beast rear legs are the robot legs, and the beast front legs are the robot arms. (Transmetal Cheetor is an exception.) The original Beast Machines Optimus Primal toy swapped arms and legs when transforming, and several beast Transformers turn beast rear legs into robot arms, produce robot legs from inside their torsos, and turn their beast front legs into kibble.
Since most altmodes (vehicles, beasts) are symmetrical, and the default robot mode is shaped more-or-less like a (symmetrical) human, the toy transformation can also usually be described as symmetrical. Anything that was on the left side of the robot ends up on the left side of the altmode, and anything done to the left side during the transformation is also done to the right side. If any parts swap sides, it's because of a full 180-degree rotation (such as the waist-swivel common in beast-to-robot transformations), with the result retaining symmetry. Minor differences in tooling between the left and right side don't necessarily make the transformation asymmetrical. (E.g., if one arm has a weapon attached, or a spring-loaded attack function.)
Asymmetrical transformation is much less common than symmetrical transformation. Examples are Beast Machines Buzz Saw (where the beast tail becomes one robot arm, and four of his beast legs become the other robot arm), Energon Omega Supreme (where the two halves of his body are entirely different toys), and Transmetal 2 Beast Wars Megatron (where the dragon's head becomes one robot arm, and the other emerges from inside his dragon body, while his dragon arms end up as kibble). Several other Megatron toys are also asymmetrical, including the original.
- Transformation (in fiction)