readable
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legible
Written or printed in such a way that the letters and words can be recognized and deciphered without difficulty; legible.
Bitte schreiben Sie Ihren Namen in Druckbuchstaben, damit er gut lesbar ist.
Please write your name in block letters so that it is clearly legible.
The word is made up of the verb 'lesen' (to read) and the ending '-bar' (-able). The ending '-bar' means that something can be done. So 'lesbar' (readable/legible) literally means 'can be read'.
Das alte Manuskript ist wegen der verblassten Tinte kaum noch lesbar.
Because of the faded ink, the old manuscript is hardly legible anymore.
'Lesbar' (readable/legible) has two meanings: physically decipherable or stylistically pleasant to read. The context makes the difference clear: 'eine lesbare Handschrift' (legible handwriting) can be deciphered; 'ein lesbares Buch' (a readable book) is interesting and well written.
Ihre Handschrift ist sehr klein, aber noch gut lesbar.
Her handwriting is very small, but still quite legible.
When referring to physical legibility, the word is often combined with adverbs such as 'gut' (well/easily), 'schlecht' (poorly), 'kaum' (hardly), or 'deutlich' (clearly). Typical nouns are 'Handschrift' (handwriting), 'Schrift' (writing/font), or 'Dokument' (document).
Das Schild war aus der Ferne nur schwer lesbar.
The sign was only difficult to read from a distance.
understandable
'verständlich' and 'lesbar' can both be used with texts. 'verständlich' says that the content is clear and that you can understand it well. 'lesbar' can first of all mean very practically that you can recognize the writing. But it can also mean that a text is pleasant to read and well written. Learners can easily confuse the words because a readable text is often also understandable. Still, the difference is important: with bad handwriting you say 'nicht lesbar' (not legible); with unclear ideas, you are more likely to say 'nicht verständlich' (not understandable).
Deine Notizen sind leider kaum lesbar, weil die Schrift so klein ist.
Unfortunately, your notes are barely legible because the writing is so small.
Der Brief vom Amt war kurz und verständlich geschrieben.
The letter from the government office was short and written in an understandable way.