Extrapolated gradients of 12 + log(O/H) along the Galactic disc for different sources. Black circles represent all the H ii regions studied in this work and the solid black line indicates the gradient obtained with them. Gradients derived from PNs (long dashed red line), H ii regions (dash-dotted blue line), B-type stars (dash-dot-dotted pink line) and Cepheids (short-dashed green line) are also represented. The Sun and the Orion nebula (M42) are shown at their respective O/H abundances located at R_G = 8.5 kpc and R_G = 8.95 kpc, respectively. Horizontal arrows show chemical abundances of the Small and Large Magellanic Clouds.