Crocheting straight edges

[PlayOnWurtz]

I'm going to scream soon. Why do my blankets and scarves always end up with extra stitches in that I then have to reduce which then expand again...I swear I meticulously count!!!
How do I get straight edges?

I put a stitch marker in the first of a row so I don't wreck it on the way back.

Are you forgetting that the first chain counts as as a stitch. So if your scarf was 40 trebles each tow begins 3 chain, 39 trebles.

and following on from astrantiamajor , you have to do a stitch n the top of the three chain.