#include<bits/stdc++.h>
#define rep(i,n) for(int i=1;i<=n;++i)
#define eb emplace_back
#define st first
#define nd second
using namespace std;
using ll=long long;
using ld=long double;
using i128=__int128;
using pii=pair<int,int>;
const ld eps=1e-12;
const int N=1e3+5,mod=1e9+7;
const int dx[4]={1,0,-1,0},dy[4]={0,1,0,-1};
int T,n,c,a[N][N];
int diff(int x,int y){
    int tot=0,now=0;
    for(int k=0;k<4;++k){
        int tx=x+dx[k],ty=y+dy[k];
        if(tx<=0||n<tx||ty<=0||n<ty)continue;
        ++tot;
        now+=a[tx][ty]!=a[x][y];
    }
    return (tot-now)-now;
}
void pri(){
    rep(i,n){
        rep(j,n)cout<<(a[i][j]?'B':'R');
        cout<<"\n";
    }
}
bool check(){
    int lin=c/n;
    c%=n;
    if(lin>=n-1){
        for(int j=2;j<=n;j+=2){
            rep(i,n)a[i][j]=a[i][j-1]^1;
        }
        lin-=n-1;
        for(int i=n-lin+1;i<=n;i+=2)
            rep(j,n)a[i][j]=a[i-1][j]^1;
        // pri();
        for(int j=n;j;--j){
            if(c==0)break;
            if(j==1&&c==1){
                if(n-lin+1==3)return 0;
                c-=diff(2,j);
                a[2][j]^=1;
            }
            int change=diff(1,j);
            // cout<<change<<"\n";
            if(change>c)return 0;
            c-=change;
            a[1][j]^=1;
        }
        // cout<<c<<"\n";
        if(c!=0)assert(false);
    }else{
        for(int i=n-lin+1;i<=n;i+=2)
            rep(j,n)a[i][j]=a[i-1][j]^1;
        // pri();
        if(lin==0){
            rep(i,n){
                if(c==0)break;
                int change=diff(i,1);
                // cout<<change<<"\n";
                if(change>c)return 0;
                c-=change;
                a[i][1]^=1;
            }   
            if(c>0)assert(false);
        }else{
            for(int j=c;j>=1;j-=2){
                if(c==0)break;
                int change=diff(n-lin,j);
                // cout<<change<<"\n";
                if(change>c)return 0;
                c-=change;
                a[n-lin][j]^=1;
            }
            if(c>0)assert(false);
        }
    }
    return 1;
}
int main(){
    // ios_base::sync_with_stdio(false);
    // cin.tie(nullptr);
    for(cin>>T;T--;){
        cin>>n>>c;
        rep(i,n)rep(j,n)a[i][j]=0;
        if(check()){
            cout<<"Possible\n";
            pri();
        }else{
            cout<<"Impossible\n";
        }
    }
    return 0;
}
/*
c:
f(n)=c*sigma(i=1..n)Ctl(i-1)*(c-1)^(i-1)*f(n-i)
f(0)=1
f(n)=sigma(i=1..n)Ctl(i-1)*f(n-i)
a[0]=1
a[1]=c
a[2]=c*(c+c-1)
a[3]=c*2*(c-1)*(c-1)+c*(c-1)*c+c*c*c+c*c*(c-1)
1 1 2 5 14
1 0 0 0 0
1 1
1 2 2
1 3 5 5
14


(n,m)=C(n+m,m)*(n-m+1)/(n+1)
ans=sigma_{i=1..n}(n-1,n-i)*c^i*(c-1)^(i-1)
ans=sigma_{i=1..n}C(n-1+n-i,n-1)*i/n*c^i*(c-1)^(n-i)
(c+c-1)^n=fra[n]*rev[i]*rev[n-i]*c^i*(c-1)^(n-i)

*/