(A) In mouse back skin, hair follicle initiation occurs via three separate waves that start, respectively, around E13.5, 15.5, and 18.5. At any given embryonic time point between E13.5 to E18.5, there is a mixture of hair follicles at different developmental stages in back skin, lacking any discernable pattern. After birth, the three separate waves synchronize into similar anagen and then telogen. Thus, it is impossible to pinpoint the exact early developmental stage of any telogen back skin hair follicle at the beginning of the lineage-tracing experiments when Tamoxifen was injected at an embryonic time point. (B) Tail skin hair follicles are arranged in triplets, and the growth of two secondary outer follicles is typically initiated next to a primary central follicle after it has already begun development. Within the dorsal middle one-third section of tail skin, at P1, the primary central hair follicles are at the hair peg stage and the secondary outer follicles on the side are at the placode stage. At P6, all three hair follicles in a triplet progress to anagen. At P15, the primary central hair follicles have already entered telogen or even the telogen to anagen transition stage, while the secondary outer hair follicles on the side are still in the catagen to telogen transition. At P21, the primary central hair follicles are in full anagen and the two secondary outer follicles are at the telogen to anagen transition stage. Scale bars: 50 µm.