One writes all its lines equal in length so that they are justified on both sides. To achieve this may involve the stretching of certain letters, though only some are allowed to be stretched, and even then they musn't be so extended that the become unrecognisable.
Stretchable letters include: Dalet, Heh, Lamed, Resh and Tav

Permitted stretchy letters

and in exceptional circumstances:Aleph, Kaf and Final Mem.

Exceptional circumstances letters

If one lengthened or shortened one line more than the one before it or the one underneath it is still kasher (valid). However only if this is accidental for if a pattern emerges e.g. a triangle or curve then it is pasul (invalid).