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tokens: along with the RP1 backbone composite series used in our plage area composite (dashed blue) and RP1 backbone composite series keeping only the days common with the RP1 individual series (dashed red). Bottom : RP1 backbone composite series when varying the included individual datasets based on their bandwidth. The different curves show the RP1 backbone composite by keeping Ba, Co, Kh, RP2, SF1, SF2, Te, Up (dashed green), Br, ML, BB (dashed blue) and Ka, PM, VM (dashed red). The RP1 backbone series used for our composite is shown in solid black. Depicted are annual median values (solid lines) along with the 1 σ asymmetric intervals (shaded surfaces). The SC numbers are given below the curves. Finally, we also test the effect of including data taken with different bandwidths in the backbone series. For this purpose, we use the RP1 backbone and produce three different versions of it: (i) keeping only datasets with either bandwidths narrower than 2 Å or broader than 3 Å (Ba, CL, Co, Kh, PS, RP2, SF1, SF2, Te, Up). Since the RP1 bandwidth is 2.5 Å, these limits imply that the chosen bandwidths differ by at least ±0.5 Å from that of RP1. (ii) including only datasets with the nominal bandwidth between 2 Å and 3 Å, that is, within ±0.5 Å of the bandwidth of RP1. In this case, we further subdivide the datasets according to whether the bandwidths that have been assigned to them; ii (a) appear to be consistent with their actual behaviour (BB, Br, ML) ii (b) or their assigned bandwidth does not appear to be consistent with their actual behaviour (Ka, PM, VM). For this test, we considered the BB and ML datasets only over the periods when the final instrumentation was used (see Appendix B ), in order to avoid inconsistencies due to instrumental changes. Figure 15 b displays the three test backbone series in comparison to the one used in our composite. All three reconstructions of the RP1 backbone series lie within the uncertainties. There are generally small differences, which are greater in 2011, before 2000, and after 2018, reaching up to 0.01 in disc fraction. 5. Summary and conclusions We processed 43 datasets of full-disc Ca II K observations spanning the period 1892–2019 to derive the evolution of plage areas over the last 12 solar cycles. We processed the data consistently with the method developed by Chatzistergos et al. (2018b , 2019b) . An extra step in the processing was added to ensure accurate results from the analysis of images from the Ky and YR datasets, which suffer from specific artefacts along arcs on the solar disc. We adapted our processing such that these artefacts can be precisely accounted for and showed that we can obtain accurate results for those datasets as well. We used our results for the 38 datasets with observations centred at the Ca II K line to create a plage area composite by applying the backbone approach employed by Chatzistergos et al. (2017) to create a sunspot group number composite. We created two backbones, one