A tally is a sum cut across a hazel stick and split down its length. Each side keeps half; only the fit is proof. Cut a sum, split it, then try to forge one half.
hold on the stick to cut: a tap is a barleycorn (£1), longer a little finger (£20), a thumb (£100), a palm (£1000) · keys 1–4 cut too · S splits and joins · after the split, cut either half alone · N for a new stick