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<jats:title>ABSTRACT</jats:title> <jats:p>The century-long (1907–2007) Ca <jats:sc>ii</jats:sc> K spectroheliograms from the Kodaikanal Solar Observatory (KSO) are calibrated, processed, and analyzed to follow the evolution of the bright on-disc structures called plages, possible representatives of magnetic activity on the Sun. This is the longest data set studied in Ca <jats:sc>ii</jats:sc> K to date, covering about 9.5 cycles of 11 yr periods. Plages are segmented with area <jats:inline-formula> <jats:tex-math> <?CDATA $\geqslant 1\,{\mathrm{arcmin}}^{2}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>≥</mml:mo> <mml:mn>1</mml:mn> <mml:mspace width="0.25em" /> <mml:msup> <mml:mrow> <mml:mi>arcmin</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjaa2c1bieqn1.gif" xlink:type="simple" /> </jats:inline-formula> using global thresholds for individual full disc images and subsequent application of a morphological closing operation. The plage index is calculated and is seen to have a close positive correlation with the fractional disc area covered by plages. The newly generated plage area cycle (from KSO) was compared with the same from the Mount Wilson Observatory (correlation 95.6%) for the overlapping years, i.e., 1915–2000. This study illustrates the time–latitude distribution of plage centroids by rendering a butterfly diagram (as observed for sunspots). The 3D visualization of the diagram shows one-to-one mapping between plage location, time, and area. This work further delineates the positional correlation between magnetic patches and plage regions through the comparison of synoptic maps derived from both KSO Ca <jats:sc>ii</jats:sc> K images and space-based full disc line-of-sight magnetograms. Regular synoptic magnetograms from ground-based observatories are available only after 1970s. Thus the long term Ca <jats:sc>ii</jats:sc> K data from KSO can be used as a proxy for estimating magnetic activity locations and their strengths at earlier times.</jats:p>