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Two offers for the same job: one says 1,750 euros a month in fourteen payments; the other, 2,041 euros a month in twelve. The first looks better, because fourteen sounds like more than twelve. Do the multiplication: 1,750 × 14 = 24,500 euros a year, and 2,041 × 12 = 24,492. It is the same salary, eight euros apart.
The misunderstanding has a legal origin. Article 31 of the Spanish Workers' Statute, the country's basic labour law, grants the right to two extraordinary payments a year: one at Christmas and one on the date set by the collective agreement. It is a right to be paid in two instalments, not a right to be paid more. And the same article allows the agreement to spread them across the twelve monthly payslips: they vanish from the calendar without a single cent vanishing.
Income tax works the same way. Withholding is calculated on what you will earn over the full year, so the extra payment is not taxed less for arriving in July. What really changes is your cash flow: on fourteen payments you take home less each month and keep two fat ones; spread out, you take home the same twelve times.
When you compare two offers, cross out the monthly figure and ask for the gross annual pay. Divide by twelve and compare. If a company only gives you the month and the number of payments, ask for the annual figure in writing before you sign.