Tune two primers to amplify the target region so both melt near the target Tm and match each other — but the two ends have different GC, so you can't tune them the same way. Graded by the real thermodynamics engine.
Position 5'→3' along the top strand. The lit band ▮ between the primers is the amplicon — the sequence that gets copied.
▮ forward primer (top strand, left end)
▮ reverse primer (reverse-complement of the right end, primes the bottom strand) · greener shading = GC-richer.
Forward primer · 5'→3'
len20 nt
Reverse primer · 5'→3'
len20 nt
melting temperature — where the two primers sit, and the gap you'd have to bridge with one annealing temp