Pickles make sandwiches taste finished. They add acid, crunch and a little bite that stops lunch from feeling flat. Start with the classics. Cheddar and sweet brown pickle. Ham and piccalilli. Tuna and gherkins. Chicken and achar.
For vegetarian sandwiches, halloumi loves sweet and sour pickle. Egg mayo gets a lift from smooth piccalilli. Falafel is better with briny crunch. If you eat meat, roast beef and pickled onions is hard to beat.
If you want a simple rule, match rich fillings with sharp pickles. Match mild fillings with bold relishes. One jar, endless lunches.