Why is SignWriting written top-to-bottom?

SignWriting is sometimes written left-to-right like English, and sometimes top-to-bottom like Chinese.

Deaf people prefer to read SignWriting from top to bottom, because they feel it is more natural to read sign language this way.

Top-to-bottom writing enables SignWriting to record a key element of sign language grammar: body weight shifts, where the signer shifts their body to the right or left as they sign.

In SignWriting, body weight shifts are written using what are called lanes, where vertically-arranged signs are written to the left or right of center in a vertical line of written signs. This can be done only when the signs are written vertically to begin with.