The monophthong chart was created in the early 20th century and has since become a widely used tool for describing and analyzing vowel sounds in various languages. The chart helps us determine the position of the tip of the tongue inside the mouth. Front vowels are found on the left side of the chart, and the tip of the tongue moves forward towards the front of the mouth. On the right side of the chart are back vowels, and the tip of the tongue pulls back.

The vertical axis represents the height of the tongue, with lower values indicating a more open jaw. Vowels that are closer to the roof of the mouth are considered close vowels, while those lower down are called open vowels.

While the monophthong chart isn't perfect, it can still provide a general approximation of the relative position of the tongue when comparing vowel sounds. For instance, the long vowel sound /iː/ requires the tongue to move further forward compared to the short vowel sound /ɪ/. It's important to note that the /ɪ/ sound is often mistakenly considered the short version of the /iː/ sound. However, the monophthong chart can reveal that the /ɪ/ sound is closer to the mid-central vowel sound, the schwa sound /ə/.

The monophthong chart isn't just used by linguists, but also by language learners, dialect coaches, and actors. So whether you're an aspiring actor or a language enthusiast, this chart can help you perfect your pronunciation.

Source: https://commons.wikimedia.org/wiki/File:RP_vowel_chart_(monophthongs).gif

Study the monophthong chart in more detail: https://receivedpronunciation.thinkific.com/collections