The first Minifigs are made in 1975, these had fixed arms and legs and were therefore also called "stiffs".
Moveable arms and legs followed only three years later.
After that many variations were released but the basic Minifig has not changed.
Regarding the parts, there are some clear differences.
These differences in parts occur in 1978, 1983, 1990 and 1998.
Changes occurred at the Head, Torso, Legs and Hands. The table below shows the important changes.
Summary of Minifig classifications by year:
Heads have the Stud either:
All old Minifigs (1975-1977) have a Head without Face (Plain). Starting in 1978 the Head has a simple Face with Eyes and Mouth. The very early faces were of Thick Pattern, but only for a short time (a year or so). The Pattern became Thin quite soon which remained the same from then.
Torsos are one of:
The first Torso had fixed Arms (1975-1977). In 1978 the fixed Arms were removed and replaced with Holes for moveable Arms. The Stud became higher to hold Body Wear like Airtanks, Jet-Packs, Armor Breastplates and many more.
Color of the Hands are one of the following:
The Yellow Hands come in four different shades. The first were of a Bright Light Orange shade, there are very few of these and found only on the very first Minfigs (1978). The second is a Light Yellow shade, found only on the Minifigs with Hollow Torso (1978/79-1982). Starting 1983 the Hands are Yellow.
Besides the Color shade there is another difference found on the pin of the Hands: a Notch, found on all the early Bright Light Orange and Light Yellow shades and on a very few Yellow. This Notch is found also on the early Red and White shades.
Legs are one of:
The first Legs were Fixed (1975-1977). In 1978 these were replaced with moveable Legs, the Minifig became a little taller too.
A minor difference on the movable Legs is the side of the Leg: found as Smooth and with a Seam vertically on the side of the Leg. The Smooth Legs are found on the early movable Minifigs from 1978/1979.
The color of accessories Vest and Airtanks are the same as the color of the Hands.
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