The voiceless alveolar fricatives are a type of fricative consonant pronounced with the tip or blade of the tongue against the alveolar ridge just behind the teeth. This refers to a class of sounds, not a single sound. There are at least six types with significant perceptual differences:The voiceless alveolar sibilant has a strong hissing sound, as the s in English sink. It is one of the most common sounds in the world. The voiceless denti-alveolar sibilant, also called apico-dental, has a weaker lisping sound like English th in thin. It occurs in Spanish dialects in southern Spain. The voiceless alveolar retracted sibilant [], and the subform apico-alveolar, or called grave, has a weak hushing sound reminiscent of retroflex fricatives. It is used in the languages of northern Iberia, like Asturleonese, Basque, Peninsular Spanish, Catalan, Galician, and Northern European Portuguese. A similar retracted sibilant form is also used in Dutch, Icelandic, some southern dialects of Swedish, Finnish, and Greek. The retracted "S" is also used in Amerindian languages such as Muscogee, Garifuna, and many varieties of Quechua. It was supposedly the standard sound of s in Classical Latin. Its sound is between and []. The voiceless alveolar non-sibilant fricative or, using the alveolar diacritic from the Extended IPA, is similar to the th in English thin. It occurs in Icelandic as well as an intervocalic and word-final allophone of English in dialects such as Hiberno-English and Scouse. The voiceless alveolar lateral fricative sounds like a voiceless, strongly articulated version of English l and is written as ll in Welsh.