Nouns like 'mar' and 'sartén' whose gender wavers but meaning stays
Most Spanish nouns have one fixed gender, so it is a shock to hear a native speaker say la sartén one day and another say el sartén the next, both talking about the same frying pan. A small group of everyday nouns can take either gender. Depending on the word, the choice comes down to region, register or number, but the meaning never changes. Mar, radio, pijama, azúcar and maratón all belong here.
English nouns have no gender, so learners tend to assume one form must be the correct one and the other a mistake. Usually neither is wrong. The useful question is which form is the normal default where you live or study, and which one is regional, poetic or simply less frequent.
The gender you choose controls everything around the noun: the article, adjectives and pronouns. Once you pick a gender, stay with it for the whole phrase. You can say la mar está tranquila or el mar está tranquilo, but never mix the two.
These nouns waver in three main ways:
la sartén in Spain, el sartén in Mexico and Central America.
el mar is everyday Spanish, while la mar belongs to sailors and poets.
el arte is masculine in the singular but often feminine in the plural, as in las bellas artes.
Rule of thumb: learn the default form of your target variety, and simply recognise the other one when you hear it.
Mar is masculine in ordinary speech. Fishermen, sailors and poets often make it feminine, and the fixed expression la mar de means very or loads of: la mar de bonito.
Calor is masculine in standard Spanish. La calor is heard in rural and popular speech, for example in parts of Andalusia, so treat it as regional rather than as a neutral alternative.
Sartén is feminine in Spain and most dictionaries, but masculine in Mexico, Central America and the Caribbean. Sazón works the same way: la sazón is standard, and el sazón is common in parts of Latin America.
Pijama (also spelled piyama) is masculine in Spain and usually feminine in much of Latin America. Radio is la radio in Spain and the Southern Cone, while many speakers in Mexico, Central America and the Caribbean call the device el radio.
Loanwords often take the gender of a related Spanish word. That is why you hear la laptop in countries that say la computadora and el laptop where people say el computador. Internet usually appears with no article at all. A lente de contacto is feminine in Spain and often masculine in Latin America.
With some words, both genders are simply correct and one is more frequent: el maratón or la maratón, el mimbre or la mimbre, la cobaya or el cobaya, la dote or el dote. Azúcar is usually masculine, but azúcar morena is very common. Words like tilde, interrogante and dracma waver in the same way.
Arte is masculine in the singular and usually feminine in the plural. The same pattern applies to arte marcial, arte popular and arte folclórico.
The most common mistake is switching gender halfway through a phrase. If you start with la, the adjective must be feminine as well: la azúcar morena, never la azúcar moreno.
Some of these words do change meaning in other uses, so don't overgeneralise. In every region, el radio also means the radius of a circle. In Latin America, los lentes means glasses, not just lenses.
Finally, la mar sounds literary or nautical in ordinary conversation. Use el mar for everyday talk, but keep the fixed expression la mar de.
A few Spanish nouns can be masculine or feminine with no change in meaning. Pick the default of the variety you are learning (el mar, la sartén in Spain, el sartén in Mexico, el arte but las artes), keep the article and adjectives consistent, and recognise the other gender as regional, poetic or simply less common rather than as a mistake.