Always handy to remember the correct space between different seeds - courgettes need bags of space, lettuce and carrots much less so. Measurements are in centimetres and inches.
This handy seed pocket ruler is a great little gardener's tool to keep to hand in the tool bag - just to check your spacings. Also the holes in the ruler allow a dibber/dibblet to be pressed through to make holes of just the right depth for your seeds.
Also available is a bigger seed ruler version.