12-month Spanish Treasury bills pay …

A short-term loan you make to the Spanish state: 3, 6, 9 or 12 months. Each bill is worth €1,000 at maturity and you buy it for a bit less; the difference is what you earn. It's one of the safest investments in Spain, because the state pays it back.

At the Treasury's auctions, held every month for each term. You can buy them commission-free on the Spanish Treasury's website (Tesoro Directo) or at the Bank of Spain, or through your bank, which may charge fees. If you need the money before they mature, you'd have to sell them on the market at that day's price.

Your earnings are taxed as savings income: 19% on the first €6,000 of savings income each year, more above that. Treasury bills have no withholding, so you pay it in your tax return. bolsillo takes off 19% to give you an idea of what you keep.

Because if prices rise faster than what you're paid, your money buys less even though you have more euros. Left in an account with no interest, inflation eats it away little by little.

From the Bank of Spain, which publishes the rate at each bill auction (the marginal rate, the highest accepted) and the daily 10-year bond yield. bolsillo downloads them from the Bank of Spain's statistics API. This is information, not investment advice: bolsillo is not a financial adviser.