Everyone has different handwriting.
The way they do their curves and lines are all different combinations than anyone else's.
Every persons handwriting is unique.
In a crime investigation, handwriting can be matched by it's characteristics. Like, let's say someone left a note at a crime scene. They'd have the suspect write a sentence so they could try to match it to the handwriting in the note.
A lot of suspects will try to change their handwriting, but usually use the same characteristics, so the investigators will still be able to tell it is their writing.
Make sense?
