The key of F major / D minor has one flat: Illustrated map of all the flat key signatures with relative minors. View key signatures on the treble and bass staves; memorize the order of their accidentals, and learn the chords found in each key.
How to Read Flat Key Signatures: 7 Steps (with Pictures) - wikiHow
An introduction to music theory: 5.6 Major scales having key signatures with flats
Lecture 5
Key Signatures Major Scales & Keys
Key Signatures: Part 3 (Layout of all Keys on Treble, Bass, Alto & Tenor Clefs)
theory - Are accidentals in the key signature and measure additive? - Music: Practice & Theory Stack Exchange
Key Signatures With Flats Key signatures, Music lessons for kids, Learn piano
Key Signatures, Key Signature
Key Signatures In Music Notation - Phamox Music