A cordon is the name given to any fruiting plant that is kept to a single
stem with fruiting spurs along its whole length.
Apples and pears grown as cordons are usually trained at an angle of
45degrees. Gooseberries produce fantastically as upright cordons and need
only the width of a hoe [ignore those who will try to tease]between them.
Red currants also. Blackcurrants cannot be grown as cordons.
Best Wishes Brian