You might know that a 40-meter dipole is also usable on 15, since 21 MHz is
an odd harmonic of 7 MHz. But as W1IS and KC1DSQ explain, you’re still going
to need a tuner for one band or the other ... unless ... unless you build their
variation that makes the antenna resonant on five bands!
During this lull in the solar cycle, 40 meters has been the best band for both local and DX contacts. Now that the sunspots are coming, other bands will soon be packed with signals as well. If you have a 40-meter dipole that's been your workhorse through the lull between Cycles 24 and 25, this will get you on 15 meters with low SWR and a minimum of effort.
If you need a multiband wire antenna and your rig’s built-in antenna tuner can’t deal with the high SWR of a G5RV on some bands, KC1DSQ and W1IS have another suggestion to consider — an off-center-fed dipole for 80 or 160 meters that will also be usable on higher frequency bands.