Speakers of the Navajo language served a critical security role during World War II. The complexity and unique phonemes of the indigenous language provided a form of cryptography used by Diné codetalkers for the US military during the second world war. The language has several consonants and consonant clusters that are difficult for non-native speakers to master. The language is tonal; vowels have high or low tone, and if long can it also rise or fall. Tone is indicated in writing by an acute accent; nasality is indicated by a mark under the vowel.