You've met them already — cerca del metro, al fondo del pasillo. Here is the full picture. Spanish has just two contractions, and they aren't optional:

	Contraction	Example
a + el	al	Voy al parque. — I'm going to the park.
de + el	del	Vengo del parque. — I'm coming from the park.

• Only the masculine singular article el merges. With la, los, las nothing happens: a la plaza, de los niños, a las ocho.
• Only the article merges. The pronoun él, with its accent, never does: la casa de él — his house.
• In writing, the El that's part of a name stays separate too: Vamos a El Escorial.

El perro del vecino — the neighbour's dog. Cerca del metro — near the metro.