Phonology is the study of the smallest contrastive units of language. These units are the smallest building blocks of language. The building blocks are organized to create words.
Linguists study how the units are organized in a language, how they are combined to form a word, and how they interact with each other.
The smallest units of language in spoken language are called phonemes. In signed language, each sign consists of the five basic parts, called parameters, in which each of the five parameters has its set of primes.
Each ASL sign (or word) consists of all these parameters: handshape, movement, location, palm orientation, and non-manual signal/marker.
Each paramater has a number of primes. For example, the handshape paramater has over 40 different handshapes (handshape primes).
When one of these paramaters (or primes) changes, a signed word results in a different meaning or sometimes meaningless (or "misspelling", metaphorically).
Many ASL signs have more than one prime of a parameter in a single production. For example, the ASL word mother+father ("parents") has one handshape, one movement, one palm orientation, and two locations.
Handshape parameter is one of the five parameters. It is the shape of a hand.
The handshape paramater has over 40 handshapes or "handshape primes" in ASL. A few handshapes, that exist in a signed language, may not exist in other signed languages in the same way that some sound patterns, that exist in one spoken language, do not exist in another language.
A change in handshape can result in a different meaning or meaningless one, in the same way that a sound unit changed in one word results in a different meaning. E.g. "bay" to "buy" in English.
ASL words school (left) and impossible (right) both have the same parameters of location, movement and palm orientation but they have different handshapes.
Location, one of the five paramaters in sign language, is where your hand is located. Eg. on your forehead, in the air, on the chest, etc. Location doesn't have its independent meaning. It is part of the sign.
These ASL words apple (left) and onion (right) have the same handshape, movement, and palm orientation, but they have a different location which results in a different meaning.
Movement is one of the five parameters in sign language. It is how your hand(s) moves. Eg. move upward, downward, backward, forward or diagonally, wave, zigzag, etc.
These ASL words airplane and fly have the same parameters except for the movement. The former one has the repeated movement and the latter one has one movement.
Another example is that these ASL words chair and to-sit have the same parameters or primes except for the movement. The former one has one movement and the latter one has the repetition.
Palm orientation, one of the five parameters of a signed word, is an orientation of the hand or palm. Eg. hands being upturned or downturned, facing you, facing away from you, etc.
These ASL words balance (left image) and maybe (right image) have the same parameters: handshape, location and movement, but the palm orientations of these signed words are different.
Clayton Valli, et al. "Linguistics of American Sign Language: An Introduction." Washington, D.C.: Gallaudet University Press. 2005. Pp 17.
Charlotte Haker-Shenk and Dennis Cokely. "American Sign Language: A Teacher's Resource Text on Grammar and Culture." Washington, D.C.: Gallaudet Press. 1980.