Are you sitting on the couch with your laptop on your lap?
You are not the only one.. a lot of people are sitting and working like this.
Why is this not a good posture?
As in the picture shows, a lot of pressure is built up in your neck and low back.
Neck muscles have to work very hard to keep your head up and they get elongated, while your pectoral muscles are shortened.
Your lumbar spine is flexed and your pelvis is posterior rotated.