This option, when checked, uses a set of different color markers to highlight groups of phonetically similar words in different languages with the same Swadesh meaning.

Phonetic similarity between two different forms is defined in the GLD as a situation in which the aligned consonants of the compared forms (usually the first two) are deemed «similar» to each other. In order for two consonants to be «similar», they have to belong to the same «consonantal class», i.e. a group of sounds that share the same place and a similar manner of articulation. The current grouping of sounds into sound classes can be found here.

Accordingly, the aligned forms undergo a process of «vowel extraction» (all vowels are formally assumed to belong to «class H», together with «weak» laryngeal phonemes), and the individual consonants are then converted to classes, e. g. dog → TK, drink → TRNK (in comparisons, only the first two consonants will be used, so, actually TR), eat → HT (word-initial vowel is equated with lack of consonant or «weak» consonant), fly → PR (l and r belong to the same class) and so on.

If both of the first two consonants of the compared forms are found to correlate, i.e. belong to the same class, the words are deemed similar (e. g. English fly and German fliegen both have the consonantal skeleton PR). If at least one differs, the words are not deemed similar (e. g. English tooth → TT and Old Norse tɔnn → TN, although they are etymological cognates, will not pass the similarity tense because of the second position).

In most cases, checking this option will highlight phonetically similar forms that are also etymological cognates and share the same numeric cognation indexes. Occasionally, however, the checking will also yield «false positives» (accidentally phonetically similar forms that do not share a common origin) and «false negatives» (phonetically dissimilar forms, not highlighted, but actually cognate). It should be noted that one should never expect this method to yield a 100% accurate picture of etymological cognacy. Rather, the method is useful for the following goals: (a) assess the amount of phonetic change that took place between related languages; (b) give a general idea of the degree of closeness of relationship for those languages where phonetic correspondences have not yet been properly established; (c) assess the average number of «chance similarities» that may arise between different languages.

The last task is particularly instructive if the «Highlight...» option is used between two different languages from different databases, i.e. not related to each other or distantly related: in most cases, it will yield around 2-3 accidental color highlights, but occasionally, the count may go as high up as 5 or 6.