Why are Spanish and Portuguese considered closely related Romance languages?

Spanish and Portuguese are considered closely related Romance languages primarily because they share a common origin in Vulgar Latin and developed within the same geographic region, the Iberian Peninsula.123

Their close relationship is supported by several key factors:

• Shared Linguistic History: Until the 15th century, Portuguese and Galician (a language spoken in northwestern Spain) formed a single linguistic unit known as Gallego-Portuguese.1
• Mutual Intelligibility: Despite differences in phonology, grammar, and vocabulary, the two languages are often mutually intelligible.1
• Common Influences: Scholars suggest that their shared characteristics may be attributed to the influence of a common Iberian substratum and a shared Moorish superstratum.3
• Geographic Proximity: Both languages evolved on the Iberian Peninsula, where they were subject to similar historical and cultural pressures, including the legacy of Roman administration and later interactions with other regional influences.453

While they share these roots, they have diverged over time; for instance, Portuguese is noted for its use of nasal vowels and a distinct verb system, whereas Spanish (specifically Castilian) is characterized by a more forceful phonetic nature and a tendency toward grammatical innovation.13